Stock output is 115 Amps.
I'm running a 3G style alternator on my 1987 (same mount as any 32v 928, including 1985). 200 amps. I'm extremely happy with it. There are higher outputs available, but you'll need to upgrade your wiring if you really need that kind of output. The main advantage to the 3G route is much better idle output performance.
I would attach supporting documents. Technical reference, step-by-step, and alternator RPM calculator. But, the Pelican forum doesn't support uploading them. So, here is where you can find them:
https://rennlist.com/forums/928-forum/1260910-how-to-mount-a-ford-3g-alternator-in-a-32v-1985-or-later-928-a.html
Or, send me your email address and I'll email them to you.