I keep about 40 clams in my 150 gallon. I would add more if there is room - because I like different colors/patterns from clams.
You can add more, but I would do so slowly. Make sure you are meeting the needs of your existing clams - before introducing new ones. More clams in a tank means that they will be competing for food (calcium, nitrate, dosing of DT's). Otherwise, light is already there - but some area will be brighter than other (directly under bulbs vs on the side).