When you select a chicken it appears a selector of how many do you want to use

i

Use the slider to select how many chickens to use.

Great.  Thanx...but why the change the old way worked fine...change for change's sake.   The new way isn't any faster and is confusing.

because some player have hundreds of chicken

Nah, they didn’t change it to “one chicken only” — the Tavern just makes you slot them in one at a time, which is a bit clunky. As long as your champ’s at max level, the chickens are the same rank, and they’re unlocked/out of the vault, you can fill all the slots needed for the rank‑up in one go. It’s been that way since the start, just not the smoothest UI.

They changed it when they did the Tavern Rework.  Brews are like that too. 

I have hundreds of chickens and brews but it is rare for me to hit the max button.  

I prefer the slider - anything in the game that leads to more convenience and/or time saving is sorely needed. 

Absolutely.  Hopefully more tavern QOL soonish.  Mass brewing please for the love of God.