As Jacko rightly points out, no official source has come out with that number.
One suspects that the "48" has been extrapolated by a journo source from the number of GR4, the aircraft that it is supposed to "replace". The requirement to provide Carrier Strike will be on top of that GR4 number, remember it was the "type" retired, not the requirement, which merely lapsed due to no suitable aircraft type.
Or so one would hope......