I started out with a small container, doing only 5 rats at a time and then I talked to my vet and by doing a whole bunch of rats together, I was saving on meds, so I got this big container and put all my 18 rats in there. I also noticed the more rats the better because it helps with their stress level.
Another plus... if you are trying to do intros, this helps a lot cause it's a scary place so instead of taking time beating each other up, they cuddle up closely together forming a bond.
Another thing I noticed, you can really tell with the nebulizing who's sick and who's got excellent lungs, those that breathing great, don't seem to mind the nebulizer, they will even play and groom themselves instead of looking for a way out.