Like anything it depends on the individual. 200 clams should last a while tho' you may want to open them to start with for your octi, not all can get through the shells, it's a learned behaviour for many octis.
Regarding variety.............I'd go for some, add in some shrimps or crabs. Also not all octis will eat clams Squidgy won't...he's a crustacean lover!
J