I used to do all this.
Certainly 7 was a figure used some years ago as a standard for long haul.
Long haul uses more per aircraft than short haul for the obvious reason that one operates all night and the other doesn't (normally). So as LCCs are still daytime-only operators they probably work closer to something like 5. They work their crews harder but as they work their aircraft harder as well the two tend to balance out for crew-per-aircraft.
A simple division is to take how many hours the fleet average per year, and then divide that by the agreed number of flying hours for crew. Then you have to adjust this for various unproductive things like training and other non-flying work, but it gives you a good start, and is where investors' consultants start from in ripping through the costs of a business up for sale.