From the limits page(1) I see you can only have 256 tables per account.
(1) http://docs.amazonwebservices.com/amazondynamodb/latest/deve...
(1) http://docs.amazonwebservices.com/amazondynamodb/latest/deve...
Edit: to clarify, I think that composite keys would make your problem limited to 256 distinct applications, not, for example, 256 customers, users, etc.