Harry - the T45 availability has been seriously overplayed in the press. Don't believe all you read.
6 Tankers is a very high number indeed - far greater than all other NATO (or most other navies) other than the USN. The need for tankers isn't as great as some make it out to be for individual escorts, so 6 is plenty to give availability. You'll only need one hull anyway due to the different operating routines of RFA vessels.