The original order of 36 3 car 730s would have been enough to cover all of the diagrams, so I am not sure why they increased it to 48?
The original plan was for the 3cars to do cross city only, at 6tph, all pairs. I don't know off the top of my head how many extra diagrams are needed to return to 6tph, but I'm going to guess an extra 5 on top of the current 11. So 16 diagrams, with a pair on each, is 32 units from a fleet of 36. Sounds about right.
The plan was subsequently changed, for class 730s to do more west Midlands local services (Rugeley, and Wolverhampton Walsall). From the post above, this is 11 single diagrams (not sure if that quite covers all services on those two routes?), so 32 + 11 is 43 out of 48 units in service.