Hydra heads questions

Hello. I’m a returning player after a decent break and am playing hydra build. I keep seeing ppl say they are doing MW Crit to get more hydra heads.

Are they actually trying to roll Crit DMG or Crit chance which somehow gives more hydra heads? Or are they tempering to get more hydra heads? Or are they finding pieces of gear with + hydra heads and trying to improve that with MW?

Also, is there a way to see how many hydra heads I get currently?