The MoD has always been careful to stress that the uptake of 138 is "over the life of the programme". I have also seen it suggested that the plan is for four front line sqns eventually.
I therefore have a suspicion that is has never been the intention to have 138 airframes at any one time - and that a final batch of very late model Bs at the fag end of the programme might replace the initial batch to see the carriers through the latter part of their lives. Hopefully I'm wrong though and in any event things can change.
I'd certainly agree that 70 or so is the minimum needed if we're to come anywhere close to using the carriers to their proper potential.