Realistically, we're going to need a fairly drastic overhaul of the squad to get back to the top. People are kidding themselves if they think we're managing any sort of consistent time in the top four, let alone mounting a title challenge, while we're still reliant on this current crop of players.
It's not going to happen in a summer, but I think three or four is pretty realistic if we're smart about our transfers. I can't remember the exact figures now but that's basically what Liverpool did with Klopp. In his third full season (which was the CL win and second place), the vast majority of the regular playing squad were "his" players (i.e. brought in after he became manager).
Unfortunately, INEOS haven't got the structure in place yet, and even when they do, there's a chance it has some teething problems, so we could be looking at five or more. There's also the strong possibility that some signings don't work out, so hopefully we're not going to continue paying daft transfer fees and wages making them unsellable.