According to "Do the Manuals and Tomes allow you to exceed 20 in an ability score? And can they be stacked?" you can stack their use to increase an ability score multiple times, even beyond 20.
So in theory, you can increase an ability score as much as you like. What practical limitations are there, if any, that would prevent a high-level character from getting an arbitrarily-high ability score via Tomes or Manuals?
In earlier editions I could point to hard limits to deal with munchkins wanting to exploit these, and I'm concerned by the lack of any obvious limit I can point at to stonewall them in 5e.