Matty,
I have the details at home.
I am not sure, from memory, but it may have been 3.
As a bomber-fighter escort on a strike mission recovery would not have been an issue. At least if we had accepted one-way missions which we, unlike the French, did not.
Not in the book, but a potentially more uesful function, would have been to tote the Gnats at a forward CAP station say 200 miles out and release them into an attacking bomber stream.
I also seem to recall that it was a plan for a Vulcan derivative, considerably larger than than the B2.
Another aircraft that would have rivalled the B70 was the Avro supersonic bomber. Again, details at home, but I seem to recall the best bit was the undercarriage. It needed 8 boggies rather than the Vulcan 4 but te plane was to jettison 4 of these so that the wheel bay would not be too large. The book didn't address the problem of arresting 8 boggies pairs travelling at 160 kts nor the problem of providing spare boggies at staging posts around the world.