i agree with candycane. don't get a crapload of them. the idea that they eat bubble algae is hit or miss. the males get large and kill other crabs at times. some may nip at corals, but i've never seen an emerald pull at anything other than xenia, which s a blessing in disguise.
there is no set number. buy as many as you like, but buy them in increments, not all at once, so you can decide for yourself if you want more.