At the moment, the odds are pretty fair that we will end up fighting Russia in Europe before 2030 so that is your scenario. The second possibility of course remains China sometime around 2028 according to the Chinese who really want Taiwan. Now in both scenarios we will be fielding maybe one flight of 6 plus our current F18s which is sad but possible so we will not be up to strength but we need to look to the future and to do that you should consider the past rather than worrying about credibility
In both scenarios we aren't fighting alone or fielding the majority of the aircraft. And neither scenario requires us to deploy a precise number of F-35s.
Some of the discussions around numbers for a mixed fleet seem to be suggesting F-35's for the NORAD role and a Euro aircraft for the NATO role. Personally I think that's backward.
It is backwards. But it's that way because the Americans will not qualify the Rafale and the Typhoon for NORAD duties. We saw this with FFCP. And if the government is intent on reducing reliance on the US, then the second fleet has to be the one tasked to NATO.
As I pointed out earlier there's a few COAs here. We can buy a bridge fleet for NATO tasks. Or we can accept a gap through the 2030s and sign up for GCAP (supposedly EIS in 2035) to start delivering in the late 2030s.