I just let my tank talk to me.
When seen needed cuc,I bought snails that could right themselves if fell upside down so got 6 trouchus snails for rocks/glass and 6 nassarious snails for sandbed and cool to watch come out and go in sandbed, and 6 red legged scarlet hermits ( read often these least aggressive hermits ) then when they couldn't keep up with cleaning tank ( along with me as I'm the best/biggest cuc member) I bought another 6 of each and 2 tuxedo urchins,bare in mind nassarious snails need meaty stuff in diet and if not enough leftovers or algae in tank on rocks/glass/sand and/or cuc members start to slow down or snails are often falling off the glass /rocks and struggling to right themselves if they can flip over then they may not be getting enough to eat so will have to supplement their diet with nori/algae sheets/algae wafers or meaty foods like clams/oysters/scallops my nassarious snails love but any meaty frozen cubed food should do,but I want cuc to have job of cleaning up so don't want them to get used to getting free meals all time and feed to often so they clean up less and less.
Once aclimnated your cuc,some maybe a little slow as they Intriduced to tank so research how to aclimintise each cuc member,I usually float bag 15 minutes,then 1-2 drip a second into container for 45 minutes and never had much problem this way,occasionally had odd triuchus snail be little slow and then a hermit picks it off and kills it as it not moving away fast enough or trying to flip hermit from its back and been toying with idea next time I get some trouchus snails then keep them in an acliminastion box at top of tank with few algae wafers/algae sheets in for few hours or even a day if needed till they adjust properley.
I read often on r2r when ordering from places like reefcleaners then get half of what is recommended for your size tank or if got 80 gallon tank then get pack for 40 gallon etc.
As if got a new tank and buy lots and lots of cuc ( sometimes see recommended in 100's) then if new tank and bot enough to feed them all and dont supplement extra food then they can start eating each other and if buying from online and got lots of snails ( 100's or even 30-40) some maybe dead in the bag and don't notice and could possibly start a chain event of catosphries,but the smell from a dead snail is absolutely terribly apparently so can easy smell it ( unless like me and got nasal polyps, then can't smell nothing lol )
Goodluck and imo buy a few at a time then re-***** and only buy more if needed