There are 20 stores you can rob in GTA Online - all of them can be done as often as you'd like (you have to wait to rob the same one again, though). Check your map for the symbols. If they have a small green check-symbol at their side, you already got this store. When you finally get #20, the trophy is yours.

It is worth noting that some stores open and close at different times during the day. So you you might not see all stores open on your map at the same time.