Make sure that you understand all of the limitations to a runtime to make sure that such a demo copy does not unintentionally disappoint your potential customers. They can't network and can't save to PDF to note two key limitations.
Since creating a runtime requires FileMaker Pro Advanced, you can install a custom menu where the new record command performs a script of your design. It can either check a counter that it increments each time it is called and exit without creating a new record when the count reaches 5 or it can check the total number of records in the table and not create any new records if the count exceeds 5--not quite the same limitation, but maybe enough as a demo ware limitation.
You can also set up startup scripts that check a date to provide a built in "time bomb" limitation.