I'd assume the limit is 12. I've never seen anyone pass 11 tools, and that was in a 2P game -- I think the highest I've seen in 4P is 8 tools.
There are 10 market cards, one +Tool card, and approximately 10-12 rounds in the typical 4P base game.
That's 21-23 opportunities for tools. Not all of them will fall to the same player, though. But someone would have to exceed 50% of the opportunities to take a tool to get there, and they probably have better things to be doing at least some of that time.