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."
Source: Transformers Live