When do you say you have enough lights?
Short answer: Never.
Slightly longer answer: Once you have all the lights you want.
Long answer (abridged): "Enough" is a quantity that varies from person to person, so there's no absolute number or universal conditions. But basically, you'll have "enough" when your purchases have satisfied whatever motivation drives your need or desire for flashlights. If you only buy lights because you need them for work, you may have "enough" after just one or two lights. However, I somewhat doubt that the majority of CPFers fall under (only) that category. So, for some, "enough" might mean one of every light that will ever be useful to him or her; for others, maybe every light with certain visual appeal; for yet others, as soon as the absolute best and brightest has been bought.
Myself, I'll have enough when I have the best (affordable within my budget) light for every situation I may encounter. I've got somewhere around 9 lights right now (not a lot, I know...), and each of them has a fairly distinct function, use or purpose to me. There are still some uses/purposes that I haven't completely satisfied yet (such as a good AA light), so I fully expect my herd to grow. And of course, since LED technology gets more advanced, I'll purchase upgrades and new replacement lights for my armada as they become available. So for me, "enough" will likely be never, since technology won't stop advancing, but short of that, I'd say I'm five or so lights away from my limit. (No, really, I am; I'm the same way with knives - addicted until utility is satisfied, which ended at about 11 knives.) (That's not to say, of course, that I won't buy "more than enough", of either knives or flashlights).
Note that the above criteria for "enough" apply only to flashaholics. I would expect the average layperson to have "enough" at one or two, or $25, whichever came first.