I guess my logic is whatever is around at the time, I flip a few bikes a year and I don't have the extra money to tie up into a "collection" per se, so my fleet rotates rather often and is usually somewhere between 2-6 bikes. With that said, I guess I like to buy/build bike types that I don't have, and make old tired bikes fresh and happy again.
I started with a target single speed, built and flipped it, then a vintage steel roadie, then flipped a few more, then upgraded to a modern alu nashbar 105, then flipped a few and bought a proper mountain bike used that I refurbed, I maintain a big box schwinn for my lady, I have an early 90s schwinn roadie I just finished rebuilding and I'm enjoying for a couple weeks before I will sell it for a small profit and put the money back into my current build, which is a mariushi roadace revival built around some RS10 wheels and a 1x7 drivetrain for errands/trailer towing.