How do you define, and possibly constrain, that acquisitive urge? Number? Kind? Condition? What makes you pull the trigger?
I backed away from too broad a definition of fountain pens to collect. I've chosen narrow areas so that I've had a few mini-collections to work on. Some are obvious; others not so much. I used to collect Spencerian pens (most made by Conklin), and when I'd sated that need, a Conklin collector happily bought the lot to cherry-pick for his Conklin collection and sell the rest. And I made a new friend. Then there's the obvious color-coded Waterman No. 7 and No. 5 pens. I collected the complete set of the black plastic ones because everyone is after the ripple versions. When that was done, I sold off all but the ones I use. I had all the sizes and colors and variations of the 1950s Meisterstucks and many of the 1930-40s flattops, but that tied up too much capital in pens that I hardly used, so those are almost all gone now. I'll keep my two favorite, though, and that's a nib thing, not a size/popularity thing.
What's Fred collecting now? Well, you know that ray celluloid that Waterman used in their Ink-Vues? I'm collecting other brands of pens that used it. Most are lower-tier Italian and German pens, and even a Wearever. There aren't many, so that keeps things under control. I have yet to meet anyone who is collecting in that parameter, which eliminates demand for the same pens. It's true that there's no boundary to my ray collection as one might determine through an old pen company catalogue, so my collection just kinda drifts along from one surprise to the next. One good friend from Hamburg Germany on arrival to the last DC show walked up to me and dropped a no-name ray pen in my hand and said "For you!" Very kind!
I'm also gathering hard rubber Crescent-fillers at a slow pace, but that's an area that can get out of control so I severely limit those to pens with great nibs.
The idea is to define a collection goal with reasonably narrow parameters. Otherwise, if you intend to collect every version of a brand, you might want to remain a childless bachelor, and with plenty of storage space.
By the way, the largest part of my collection is defined less by brand or vintage, but rather by their usefulness; their nibs are stellar. You see, I'm one of those degenerates in the pen community who's called "a user."