From what I've read, tactical ability between different players starts to equalize at about the Expert level (federation OTB ratings, not online!) ....from that point on, other pattern recognition skills (non-tactical) kick in and all these other skills this game requires (positional understanding, opening theory, endgame knowledge) start to become the more deciding factor.
In other words, a 2100 vs. 2200 contest is not going to be purely decided by "who's the better tactician".
So to answer your question => as many books/puzzles as it takes to get to an Expert rating level at Federation/OTB games. Sure, you need a lot of other types of studying to get to this goal, but one would assume you'd be cracking the tactics books a heck of a lot until that point.
Suppose you buy a big book of 1500 chess puzzles, starting easy and getting harder and very hard at the end. You do them againa and again until you have them all memorized. They are taken from real games. Then you buy another 1500 book and repeat. Each position is different in some way.
How long do you think it would be before your skill level peaks, or your growth rate slows? 1 book? 2 books? 5?