I’d start with 10 or so. Vermetid snails populate faster than bumblebee’s eat. I hate to say it but you’ll probably have vermetid snails forever. My overflow and sump chambers have vermetid snails but display has near none.Got 3 bumble bee snails for my waterbox 20 hope it’s not too much, i constantly shift my sand and it’s so annoying seeing all that web all over the place also when I feed