Kbrock,
Good question - hope I can help out here.
CONOPS are always a moving target, but at the time when I worked the programme, the USMC was looking at embarking roughly the same number of F-35s as AV-8Bs on LHDs - which meant around 6, not 20. At LM, we did look at deck layouts for larger air wings, but as far as I know (and I could very well be wrong) embarking 20 F-35s on an LHD would not be a normal event.
You're quite right that an F-35's fuel requirements will be different to an AV-8Bs, but how different I couldn't say. They will very probably use a good deal more. In any case, a ship operating aircraft like the F-35 (even a CVN) will have a tanker in closed company.
Deck spotting factors are one of those things that the USN has made quite complicated, but with some reason. Length and span dimensions are larger wight he F-35, but spotting aircraft on a deck (and being able to move them around) also relies on location of landing gear on the aircraft, turning circles, and so on. Where overall dimensions really count is when looking at elevator spotting: the F-35 requirement specifically excluded a requirement to fit down the aft elevators on the earlier (already obsolescent) LHD class ships, so as not to force a folding wing.
Generally, the overall dimensions of the aircraft were driven by the need to get the weight inside the envelope that a STOVL aircraft could handle. Compliance with the required flight deck spotting factor basically happened as a by product of the weight drive. (We also had a 'maintenance box' requirement to meet).
What I can say as a fact is that the F-35B can operate from an LHD in the numbers required by the USMC. And yes, it's essentially on a one for one basis, but not at a hypothetical figure of 20 aircraft.
Hope this helps
Best Regards as ever to those shifting the jets around the decks,
Engines