After having played the single player campaign extensively, I just took my first foray into mulitplayer, and I was surprised that my renown went up after winning. I had originally assumed that the maximum possible renown was 17 (which you can get through the single player campaign). But what is the actual maximum possible renown, when increasing your renown through multiplayer?
I reported this as a bug just the other night. It seems your max renown is 17 in sp - and 18 in mp. Sounds like an oversight, since renown is supposed to emulate the highest cleared sp content. That being said, the current (if possibly unintended) renown is 18.
Word from the devs: Not a bug, rather than the renown cap for Single Player is lacking in content to get us to 18 (for now). Wink wink.