I love these little snails I have two. I have a 58
gal tank with 14 snails of various parentage.
IMO get a variety they all prefer certian algeas and detritus. The bumble bee snails I love because they get in the nooks and crannies of the live rock and dont seem to create as much waste as my Astra snails. Most people have more snails then me though. If I see the need as my tank contiues to mature I will add more as needed. I would say get ten to 15 snails and then see if you still are seeing algea. If so get a few more accordingly. I do not have a
DSB and they seem to be doing just fine!
HTH