Nine aircraft may not sound like a lot, but it really depends upon availability.
I flew around 6,000 hours on the Nimrod, a fair proportion of those with important bits of kit either not working or not even present in the first place. Some days, out of the serried ranks of aircraft sat out on the Line, only very few were truly serviceable.
I now have 5000 hours plus on the 737, and instruct on the -800 and guess what? Nearly every single time you get into one it works as advertised! Now of course we don't have mission equipment to worry about, but I would be prepared to bet modern reliability is an order of magnitude better than that of previous generations.
Looked at in that way, you should get a pretty solid capability from 9 modern well equipped jets - just as long as the good old RAF does not fall into the trap of over engineering every aspect of maintenance and support that is.