You will need very good lighting and excellent water quality for clams. If you have that then no problem. Depending on the type of clam you are looking for they can range from about 30 to over 100 and I'm sure more.
DT's Phytoplankton is IMO, the best food for clams along w/ BRIGHT light. Keep calcium levels up (400 at least) They are said to benefit from maintained Strontium levels but w/ light, feeding and calcium i have had good results.