In various reports online Roberto Orci, member of the Transformers II writer triumvirate, wrote on the Don Murphy boards that the TF cast will be 20 robots in total, about 10 on each side.
Who those ten will be remains a mystery. It is probably not a stretch to assume at least 7 of them (Optimus Prime, Ratchet, Ironhide, Bumblebee, Barricade, Starscream, and Soundwave). Jazz remains a question mark to me.
Orci's post is here, now that Don Murphy's site is back up.
"I'd love and think we could almost barely manage ten on each side, with some front and center and others as more supporting."
Interesting choice of phrasing in that quote. Rather then saying there is 10 on each side (for 20 total), its phrased more as a preference or desire. Basically that means the number of total TFs in the movie remains unknown with 20 just being a ballpark number.
Update: From a post today, Orci pretty much confirmed that the 20 Transformers cast was more of a wish then an exact number, "it's kinda maybe not entirely accurate whatsoever."