Disclaimer
Take the deckbuilding advice in this description with a grain of salt. I have not actually played Abzan CoCo before; this is my analysis of the following cards and combos based upon (what I hope is) a logical approach. All this is derived mostly from thinking and knowledge of the Modern format, rather than testing and personal experience. That being said, I hope you will read it and that it is helpful to you! All suggestions and dissenting opinions are welcome.
What is the goal of this deck?
This is a Abzan CoCo deck testing out Vizier of Remedies + Devoted Druid. While there are many directions to take this combo, this deck approaches it from the perspective of wanting both an aggressive angle and a combo angle, rather than one or the other. The trade off is that a fusion of the two will not be as strong as a deck with a singular focus. It is my belief, however, that a two-pronged approach is more flexible, resilient, and trickier for the opponent to neutralize. The following analysis assumes this type of strategy, as evaluation of the same card can differ based on the goal of the deck.
Eternal Witness + Chord of Calling may not be necessary for finding extra copies of a removed combo piece mid-combo. This requires infinite mana and Duskwatch Recruiter, meaning your opponent must stop the combo before infinite mana is generated or counterspell the Duskwatch Recruiter. After that point, you can just combo off in response, so no Chord of Calling for a removed combo piece is necessary.
Multiple copies of Eternal Witness + Chord of Calling can be used to get the persist infinite life/damage combo (Murderous Redcap and Viscera Seer; Kitchen Finks can be cast), thus negating the need for Walking Ballista. The downside is that it requires a Chord of Calling in the hand or graveyard. Collected Company can also fill in for Chord of Calling; stacking the deck to put the combo pieces in the top 6 cards. I'm not sure if this shortcut actually works; Duskwatch Recruiter looks at 3 cards so it would take a while for a top 6 to contain both combo pieces, but I believe it must happen eventually, given infinite iterations. Alternatively, with two Eternal Witness, each one could return a Collected Company to find one combo piece in the top 6, which is a definite occurrence. This would work well with a "draw a card" creature to stack the deck to a Chord of Calling or Collected Company, negating the need for it to be in the hand or graveyard when the combo starts. Elvish Visionary seems like the best option.
Both options are bad Collected Company hits.
Both win the game on the spot with the infinite mana combo. Murderous Redcap and Viscera Seer can be tutored into the battlefield with infinite mana and Chord of Calling or Collected Conpany, as opposed to the previous Viscera Seer + Kitchen Finks, which scryed Murderous Redcap to the top to win next turn.
Murderous Redcap puts a strain on the mana base if/when it actually needs to be cast (although maxing out Birds of Paradise helps). Alternatively, using Chord of Calling circumvents the mana cost. Getting a Chord of Calling during the combo turn would guaranteed with Elvish Visionary, but that means taking up one more slot.
Walking Ballista is more able to be cast for value; Murderous Redcap has a similar effect, but more constraining mana cost.
Walking Ballista doesn't use the graveyard, negating what was a weakness of the deck. Would be better off in the sideboard, as maindeck grave hate is unlikely? Or would that just take up a slot in the sideboard that needs to be switched out for post-board games? In that case, it would probably be better to have it maindeck.
The infinite mana combo with Duskwatch Recruiter and Walking Ballista gets around Grafdigger's Cage, which fetching up Murderous Redcap and Viscera Seer using Chord of Calling or Collected Company does not.
Overall, I think it is better to run one or the other to cut down on the number of cards Collected Company does not hit. I also think that Walking Ballista is the better option, as it gets around cards like Rest in Peace and Grafdigger's Cage which the deck was previously weak to. To clarify, the deck is still weak to both, as shutting down Kitchen Finks or Collected Company hurts, but this combo can gives a better possibility of beating those cards. Additionally, those are the most played graveyard hate in the format; to truly stop every combo in the deck, one has to resort to lesser-played sideboard cards like Torpor Orb and Linvala, Keeper of Silence. In the vast metagame that is Modern, players may be reluctant to run such narrow hate cards (assuming this deck isn't totally busted and necessitates an increase in those cards).
What is the optimal number of each new combo piece? Factors in determining the numbers should include individual card quality and function, the frequency at which I would like that card to appear, and overall synergy the card has with the deck.
How many Duskwatch Recruiter?
Duskwatch Recruiter is the key outlet for the infinite mana combo. This means that the better/more frequent the infinite mana combo is, the more copies are warranted. Increasing the number of copies increases the odds that it is in opening hand/top 6 cards for Collected Company, making faster combo more likely.
Duskwatch Recruiter on its own is not a bad creature. It can act as a generator of card advantage in the late game, and the werewolf side can cast dorks and can beat down, which is the alternative to combo. However, it has rather weak p/t, and it's transform ability can be hard to control, especially if I want to leave up Collected Company or Chord of Calling. In addition, the opponent can try to manipulate the transform ability to deny access to the combo by not playing spells. This means that a Duskwatch Recruiter sitting on the battlefield waiting for other pieces of the combo can be a liability compared to it being the final combo piece.
Overall, I think the correct number is 2-3. As the last combo piece I want to play, Duskwatch Receuiter is tutorable by Collected Company and Chord of Calling with infinite mana. This means that there are more copies of Duskwatch Recruiter during the combo turn itself, so less copies can be ran.
How many Devoted Druid?
Devoted Druid is both mana dork and combo piece, making it highly versatile and important for the deck to function. Chord of Calling in particular needs the mana acceleration. In addition, it is has synergy with Gavony Township.
The obvious comparison for Devoted Druid is Wall of Roots. Devoted Druid can't block as well, survive Lightning Bolt or Searing Blaze, has summoning sickness, and cannot both add mana for and convoke a Chord of Calling. However, Devoted Druid can add multiple mana per turn, after summoning sickness, works with Gavony Township and (quite obviously) is a combo piece.
Devoted Druid will likely act as a lightning rod; "bolting the bird" is a common adage. However, this can be seen as good, if it draws away removal from other important creatures.
While it has less p/t than Wall of Roots, the lackluster ability to block shouldn't be much of an issue. Against truly aggressive decks like Burn, grinding with Kitchen Finks is as good a plan as combo. Additionally, a damage spell going at a creature is not going to the face.
Overall, I think the correct number is 4. It is the piece that will most likely be played first, and will suffer removal in response to playing Vizier of Remedies (though Vizier of Remedies is more a "true" combo piece) in order to prevent infinite mana from being generated. Outside of combo, mana dorks have been, and will continue to be, important to any CoCo deck.
How many Vizier of Remedies?
More than anything else, how often one wants to combo will be the deciding factor on the number of Vizier of Remedies one runs. Unlike the other pieces, it does not have many uses outside of combo. Additionally, it is has a weak p/t. That said, it is integral to both the infinite mana and infinite life combos.
Overall, I think the correct number is 3, with 1 Anafenza, Kin-Tree Spirit. While Anafenza, Kin-Tree Spirit does not combo with Devoted Druid, it is a more powerful card on its own and sets up infinite bolster with Kitchen Finks, allowing another win condition besides Walking Ballista that can be found with Chord of Calling or Collected Company. Post-board, it acts as protection against Surgical Extraction effects. However, it can be removed for the 4th Vizier of Remedies depending on whuch combo is better/appears more frequently.
