I hate to disagree with you but I wouldn't go as far to say," As many as you want and have room for".
You might take a glance at Daniel Knop book, " Giant Clams". ( Page 177 ) He states that clams lead to fierce competition are they need a certain amount of substance because of their intensive growth rate, calcium for one. Monitor your calcium closely as the more clams you have the more calcium is needed for growth and ofcourse amble food.
JMO.
Barry