My own personal method has basically two requirements:
1. Any car I have has to be either drivable or I am actively working on making it so (exception: I did keep a rust-free e30 shell for a few years as a backup in case I crash the rally car and need a shell to swap everything into to rebuild)
2. I try not to every have two cars that "do the same thing" or "are the same"- because what's the point of that? Ex:
- The GTI is my daily driver (needs to be comfortable, reliable, good MPGs, nice enough to take coworkers to lunch in, etc).
- The Sequoia is used for towing the trailer and hauling large objects, and other utility tasks.
- The Porsche is straight-up a road-trip car, and ultimately intended to drive cross-country with. It could also do some track days.
- The e30 is a competition rally car. Jim (my rally codriver) has another e30 set up for track that I drive with him when we go tarmac racing, so I dont' need my own "racecar" for the track (thankfully).
- The Raider is for rally reconnaissance and general off-roading (unlike most Jeeps out there, it isn't a pavement queen), and occasional light utility stuff. Not for DDing.
I do buy parts cars on occasion, but they only stay around long enough for me to take all the parts I want and then haul them to the scrapper. Usually counted in weeks, not months or years.
The reason I don't have any other projects at the moment is because I can't think of any automotive "missions" that I don't currently have covered. Not so say I won't find another project car at some point, but it will have it's own distinct reason.
I honestly dont' get some of you who have 6 Miatas or 27 Boxsters or whatever lol.....I had a rust-free e30 shell in my possession and had no interest in doing anything with it, since I already did an e30....
The downside to my "everything different" plan is that almost no parts crossover, and of my 6 vehicles, none of them can use each others wheels - so I have a crapload of wheel/tire sets lol. At least all of them use metric......