How much is a sleeve of celery?
This question is actually answered in a comment to the recipe you linked:


  kimberly says:
  
  Sorry for silly question, but the ingredient list says ” ½ sleeve
  celery $0.65 “. do you mean 1/2 of a stalk or 1/2 of a bunch of
  celery? [...]
  
  Beth M says:
  
  Yes, I meant a half bunch. Sorry about the confusion! :) I used about 4 stalks or so.
  They cook down quite a bit.


Also there's a picture of the ingredients used: 



So 4-5 stalks appears to be intended amount of celery. A recipe like this though is going to pretty flexible so if that seems like too much or too little it shouldn't be a problem if you go outside that range.