The DX7 was 6 operators, and the module in the CX5M (I had one) was 4 operators, like the DX21. It used the YM2151 sound chip, which was also present in a number of video games.
This was an awesome computer, very fun to play with.
Aha, thank you for jogging me grey matter, I had one as well (got one with a couple of friends and formed a band - one gig wonder). Had a TR707 as well, fun times and great intro into music. Did enjoy the CX5M immensely, though went out of my way to avoid any preset sound. Once you've owned a synth, you just recognise all the presets in all the songs, but was easier back then unlike todays sample world.
I did prefer the bass sounds on the Casio Phase Distortion synths though over the FM options as sounded richer (at least for me).
This was an awesome computer, very fun to play with.