How high can a 'sub floor' be raised withh plywood sheets?
You can, but it's an expensive way to go, usually.  And plywood is an inferior top surface for tiling onto.

If not simply re-doing the "mud" approach (worth considering - works well, may cost less) you could use (top down) 1/2" backer board (set on thinset), 3/4" plywood, and set the plywood on 3/4" strips spaced 6-12" or so apart, rather than two full sheets of 3/4" ply.