[|Erasmus|]

im not a good protoss player but, I think the general concensus is if you didn't open stargate you should never go phoenix to counter muta. It takes you so long to get out the stargate and start phoenix production that you will just fall behind due to how zerg production works and you will probably just keep losing small groups of phoenix to mutas unless you have godlike micro.

if the zerg is going mass muta, invest in 2-3 cannons per base, get blink and attack upgrades for stalkers, and tech to archons and tempar w/stom. Then because zerg will never kill your army, make 50 cannons so they can't base trade and push across the map (maybe exaggerating a bit, but it's sort of how it works if they got a really big muta flock).