Depending on how many fish you've got I'd say that you should be ok for 4 or 5, remember that they are opportunist hunters and as such will eat any fry they can get their claws on (just to warn you, if you are going to try breed any or your fish).
Make sure the shrimps that are in your lfs are not in brackish water, if they are you might try putting them in via the drip method, of try another lfs that keep theirs in fresh water.
70p for 20 is cheaper than I get them (though I'm afraid mine are for food, though they sometimes live a while).