When steaming clams you can steam as many clams at once as you can fit into the pot.
I usually steam about 1 lb of clams at one time unless I am serving more people then I steam as many clams as I need at once.
If you are needing to steam a lot of clams for a party, gathering etc then you can steam them all at once using more pots on the stove.
Don't overfill the pot with the clams but allow some room for them to move around while steaming them.
I fill the pot to about halfway with the clams and then fill the rest of the pot to near full with water.
However you don't want the water to boil over in the pot when steaming the clams so don't fill the pot completely with water.
Just add enough water to cover the clams and then allow them to steam after bringing the pot to a boil then turn the pot down so that it's not boiling.