Group them by common functionality - for example, place all your database i/o interfaces together.

Generally, if your interfaces are reasonably well abstracted (in other words if your interface is 'self-sustaining' and does not depend on the metods & properties found in another interface)you should have quite a few dll's if you expect to have to change things.

By the way - if you expect change just add interfaces like:
Implements IInterface
then later:
Implements IInterface2

When you change it add an interface do not change the old one:
Implements IInterface2 - add the new interface in same same dll as the old one.