I drop in shrimps one by one, until they don't eat anymore.
After that, I drop in a couple of pieces extra, because I noticed my reds like to scavenge and search the gravel for scraps a bit after having dined (it's really cool to watch them screening the bottom, nose down, moving gravel, and chasing off pleco's from the leftovers they're munching on)... The leftovers are taken care of by my cleaning crew (pleco's, raphael cats, cories). I never remove any leftovers, since they don't last lounger than 60 minutes...