Cuc?
 

I have a 90 gallon taank and a 30 gallon sump, the tank is cycling and it is time to think about a clean up crew, just wondering approximatley how many snails etc I will need

when i added my cuc lil over a month ago i was told 25 or so snails and add more if needed if you add a 100 more than likely half will die from stavation thats in a 120G

Depending on the snail you'd go with so many per gallon. I think for Trochus for example it's one snail to every 3 gallons or something stupid like that. In my 90 I have 20 Trochus, 15 Nassarius, 15 Cerith and 15 Astrea...
well that's what I started with, there might be one or two of each that have died...

I agree, what you typically see posted as a number per gallon is too high. In my 90 I have maybe a dozen Astreas and half a dozed Nas. I also have a sleeper goby to help with the sand bed.