It all depends on the intended function of each critter. Some are there to eat algae on the glass, some are there to clean the sand of detritus, some are there to clean the algae in the sand bed. So how many of each depends on overlap. You wouldn't want 30 each of 4 different algae eaters and no detritus or scavengers.
I am one for starting small and adding if they aren't doing the job. That allows me to decide which functions needs to be supplemented, it also prevents loss due to starvation if you have too many of one particular type. I would rather add to achieve balance rather then let living things die.
I started with a TOTAL of about 100 critter in my 90. Astreas, Nerites, Hermits, Nassarius, Trochus, Ceriths. Then added a few larger things like a Fighting Conch and a Sally Lightfoot Crab. I will definatlet be adding some more Trochus and Nerite snails. These seem to be the best glass workers and can right themselves when the fall.
FYI, We did a bulk order from reeftopia.com a month or so ago and I was happy with the mix of things we got. Even if you aren't buying bulk, the prices aren't horrible. Premiumaquatics also has a nice selection and they will ship 2 day ground to us, I have had success with them also.
Dave
if food is slim in the tank just target feed
