I understand the permit to fly is valid for 14 days, but how many flights can be made in that time?
Assuming you mean a temporary Permit to Fly (rather than a 'permanent' one) for an aircraft normally flying under a C of A, then this site states that:
You will be required
- to supply a validity period while completing the application.
- Temporary permits are usually issued for less than a month, but you should allow enough time for the flight to be conducted, taking into account weather conditions etc.
I'd guess it is intended for an aircraft to be ferried in order to renew its C of A or at the end of its operational life?