If you are using the NUnit GUI, You must enable TypeMock First.
You can do this by creating a shortcut to:
[TypeMock-Install Directory]TMockRunner.exe [NUnit-Dir]in
unit-gui.exe <path to dll>
NOTE: The visual studio plugin will work only for executables run from within the studio. So if you change the debug application of the test dll to run nunit-gui, you don't have to run it through TMockRunner.
To answer the other question, if you don't use the demo there is no limit on the amount of types mocked.