Room with space for 3.... that's how it starts lol. My first two were woodies (generic, dynamos) in my apartment around 15 years ago. I've moved a few times since then, but there's always a battle of trying to pick up neat things and talk myself out of stuff that I just don't have the room for. I don't know how many I have at the moment- and I still find things I really, really want but can't fit. If I had to pick, it'd probably 3 boring ones. 1 Japan universal (Egret 2- hori/vert), 1 MVS (Neo Geo), 1 Midway (Ki/MK/ETC). This would net me the majority of the jamma library with the option to go with some gun stuff if I wish.
The downside is that I miss out on the gimmicky stuff going this direction so picking just 3 would be really tough. Police 911, DDR, Darius (WS), 4 player cabs, driving cabs, etc.
Pain in the butt having to pull them out to get to them, but each one serves its purpose and I can't see myself getting rid of them any time soon.