It's generally up to the airport operator to decide what level of service they want to provide. Although the CAA does have the power to tell an airport to provide a particular level of service I don't think it does it very often.
It's less to do with what goes on at the airport - except that to accept commercial operations an airport will have to have ATC or FIS - and in most cases comes down to the workload and safety. There is no magical figure, it will depend a lot on the layout of the airport, e.g. lots of runway crossings would be a good reason to have FIS.