Like anything else, priorities in life are the first factor in such a decision. Would I rather have another firearm, or would I rather have ___?
I really don't see myself owning more than 75, or probably even 50, even though there are so many out there that I would "like to have". Since they hold their value better than other things like electronics, shoes, dresses, etc. (these are just examples--I'm not REALLY poking my wife in the eye) it really doesn't make sense to get rid of any just for the sake of doing so... but at the same time, maintaining a big inventory takes time and effort, and everybody knows a 48 gun safe really doesn't hold 48 long guns, so storage is also at a premium. I have rifles I haven't fired in several years, but will certainly gravitate back to someday (after this current deployment is over!), and I have others with sentimental value that haven't been fired in longer than that that i'll NEVER part with.
I currently have just over 30, and when I fill the most glaring gaps in my collection i'll probably be okay at about 40. When I get older and shooting less frequently I probably will start gifting some of the less sentimental ones, and saving the special ones up until the very end.
hopefully that day is about 50 years down the road...