A couple of medium size should be fine. How much rock do you have in the tank? Is it cured? If you've just started and added cured live rock, directly from a fish store, you don't even need the raw shrimp step. If it's uncured rock, you don't need the shrimp cuz as the rock cures you build enough bacteria. Long story short, if you have much rock, I'd skip the step. If you don't and want to cycle the tank for fish only, then a couple shout do.
Good Luck!