I went through a spurt in the early 2000s when I was getting 4 or 5 a year, when the A list Bally/Williams games were still "affordable". I'd just save up and get one. Now, it's mostly opportunistic - if something cheap and close pops up, I'll snag it, whether to fix up/sell or keep, depends on the title. After 20+ years I've pretty much got my core games, and rotate others in and out. I average 2 a year now.
As stated above, once you get over 8 games or so, maintenance and upkeep become a consideration too. Personally, our games don't get played much. We only have a couple parties a year, and then it's just myself and my wife, basically a game here and there. With 20+ machines, I'd be surprised if any particular one gets 6 or 8 hours of actual play annually. In fact, every few months I try to go from one end of the lineup to the other just to keep them loose. Consequently, if I shop one and get it working 100%, it's not gonna need much more than wax for a long time.
Long story short - don't borrow to buy, and get as many as you want whenever you want, as long as you're taking care of real life first. Don't overpay, take care of them, and you'll get your money back.