Yeah, what you're looking for is Full N-Key Rollover (or at least Alpha N-Key Rollover).
I tried to get a Model M with custom electronics made for Full N-Key, but it would cost me a ton and I'd have to make a bunch of keyboards and then sell the rest for it to be anywhere near feasible.
The Apple Extended II Keyboard is Full N-Key, but it's on an ADB connector. You would get at most 6 keypresses at the same time if you managed to find an ADB-to-USB connector, and I can't find any ADB-to-PS2 connectors at all.
I think only the Razer Tarantula offers you Full N-Key Rollover nowadays for gaming. The Logitech keyboards weren't good for that.
I just returned a Steelkeys 6G keyboard because, even though it had great tactile feedback and typed very cleanly (it uses Cherry switches), it doesn't have Full...