id say start off with 3 and see if they live. burping is where you put the clam in a container of water and wait for it to release any water it has inside it, so that the clam doesnt expel this water into your tank. people do this so the clam doesnt bring bacteria or something bad into there tank.
and yea, just buy like cherrystone clams or something from the grocery store, they should be like 30 cents each