what is the most snails and most crabs i should put in a 10 gallon salt tank?

Apparently no one noticed you specified this was a SALTWATER tank. Yes, you should have snails as part of your cleaning crew. Saltwater snails aren't the reproducers that nuisance freshwater snails can be.

I've always kept around one snail per gallon of water, although I know others who keep up to two per gallon. If you're getting these to actually control algae, get a few different varieties, since you'll control more types of algae this way. Some that I'd definitely recomment to keep are nassarius (this one is a sand sifter and doesn't eat algae, but aerates the substrate and eats food other surface dwellers can't), Trochus, Nerites, Turbos, and Ceriths. Margaritas prefer cooler water than is found in most saltwater tanks and Astraeas have a bad tendency to fall of surfaces on their sides - they can't right themselves, so if you don't turn them over, they die and affect your water quality.

Crabs are "iffy" depending on what else you plan to keep. A 10 gallon tank has only enough room for one small fish (shrimp or clown goby, or a firefish) and almost any crab will eat small fish they can catch. If you plan to keep corals, anemones, or macroalgae, some crabs will eat these as well. So I'd advise doing lots of research before you add any. A porcelain crab (which isn't a true crab) would be one of your safer choices, as would blue-legged or scarlet hermit crabs (just add extra shells so they can "change" as the grow - if you don't have any extra shells available, you may notice you have fewer large snails in your tank!). Apart from those, emerald (or the red version of the Mitrax crabs) or Sally lightfoot might be the safest. Arrow crabs are cool looking (like an alien), but these might be more of a problem. I wouldn't suggest more than two, and I'd try different species to prevent aggression between them (except for hermits, which you could keep about 1 per gallon again).

No snails because they will devour your tank and a crab or two would be fine for a 10 gallon. If you get any snails, get one because in a month you'll have five or six.

I would only put 2 crabs in, and NO SNAILS! I accidentally took a snail home from the fish store, and the darn thing laid eggs everywhere. When the eggs the fish didn't eat hatched, THEY laid eggs. I called the store to tell them that I had one of their snails when I first got home, but they had told me to just keep it. Now I know why!

if u don't want snails, u could get from your Local Fish SHop, some shrimps and add to the tank, that would help to control the number of snails.

since the shrimps would eat the snails too!

:) amazing sight. trust me.