I refer to myself as a pen accumulator, rather than collector: I try only to buy pens that I will use, and that are not so precious that I will be heartbroken if they get a little dinged up from traveling in my bags. That said, I've developed a fondness for vintage striated Sheaffer 375s, and now have one in each of the colors (silver, tobacco, marine green, carmine, and the much rarer roseglow striated). I'm trying to convince myself that I am not going to collect Sheaffer Targas, even though they're really comfortable, they have the great inlaid nibs, and they come in so many amazing finishes. Two Targas, even three, is okay; four would lead down the slippery slope of collecting.
That said, I have around *mumble* sixty fountain pens, and am culling my collection (Spika, check for a PM from me), hoping to get down to thirty pens that I will actually rotate through on a regular basis. I carry about six with me regularly: two Parker Sonnets, one Sheaffer Legacy Heritage, one Cross Century, one Waterman Ici et La, and one Pelikan 200 (in the rare Telekom magenta-and-grey marbled finish) -- all of which have customized super-needlepoint and tiny cursive italic nibs.
As Jamesaritchie observes, fountain pens will last. My oldest pen, as far as I can tell, is a roseglow striated Sheaffer 375, from the late 1930s. It's had its sac replaced once, but still works excellently.