I think he'll get more than 18 for a few reasons. Some will argue that the passing attempts will be limited next year and that there won't be alot to go around. The Seahawks averaged 25 attempts in 2012, but that's not counting the passing attempts that turned into running opportunities. To be safe let's say 50% of Russell Wlson's rushing attempts were designed passes that he decided to tuck it and run. That puts you at 28 actual attempts per game. Assuming Sherman is correct and RW is much faster in getting through his reads and making decisions, I see no reason why he can't attempt 30-32 passes a game. If he's averaging 32 a game, and if he keeps his completion % around 62% (he was at 64% last year) that would mean you'd have 20 completions a game. It's not inconceivable that he could get 1-2 receptions on average per game.
I thing they'll be creative and design ways to get Willson the ball especially near the redzone where he can use his size to make plays. I'm also excited to see what kind of impact Harper will have, again as a redzone target to go up and get the ball I think both of these guys could be dynamic offering size that we don't really have at WR aside from Rice.