Boslandew
No doubt someone with more specific knowledge can give you a more accurate answer, but in the interim consider these points:
I think the average RAF fighter Sqn has 12 aircraft, but I think they may be aiming for 16 per Typhoon Sqn. (60-80)
The OCU may well have 20+ airframes, and there is an OEU too. (20+)
Attrition. If the Typhoon fleet is to last 30 years, and on average you expect to lose 1, 1.5, 2 a year for that period then you need to buy 30, 45, 60 above your minimum requirement not to lose capability as you lose aircraft. (30-60?)
Not all airframes are "in use" by a Sqn/OCU at any one time, some are in deep maintenance, being upgraded, etc.
I believe there is an attempt to rotate aircraft around the fleet to spread the usage, so you don't get some aircraft with lots of hours on them, and some with very few...
No doubt someone can give you a better answer!