At some point you'll reach a critical mass where it doesn't hurt to sell one or two because there's always an upgrade or new shiny out there to replace them. So for me its not the number per se but what they are and how they fit into the family. Another great thing about that critical mass is that at some point, Her Ladyship ceases to ask you what you're doing when you walk into the house carrying a rifle and she definitely can't figure out if its new or just a different year's vintage of your SMLE's, Mosins, Mausers whatever - that means additions are not an issue.
But, on the serious note, there is that investment issue. What does it cost in time / organization and money to insure them? Can you afford to not insure them? Is what you love under threat of confiscation (along with then orphaning accessories, ammo whatever)? Do you have adequate insurance?
I'm actually a bit peeved (rhymes with dissed) about the long term possibility of confiscation of anything but a single shot .22 short because I'm just at the end of a 1.5 year (planning and construction) addition to the house that included a purpose built gun room. I've now got a play space and it looks increasingly that someone will eventually want all my toys ..
OK so short answer to the question - how many are enough .. when you can leave a gun show without buying or lusting for that whatever's your passion, (and you can realistically afford / feed / house something if you bought it,) you have enough.