Yeah, 10 matches unless you're so incredibly 50:50, in which case you may play a few more until it places you, for example I played 11 games.
Simply put, if you've never played ranked before then you have a hidden rating of 1200, and when you lose or win games during your placement games your rating will go up by +/- 49 points for each game. This is all hidden until you're placed, at which point you can see your rating. From then on the system will try and make you win 50% of your games, so if you're winning more than that then you'll gain rating, and if you lose more than that you'll drop rating, until you're playing with people of a similar level.
The problem with all this is that you make up a tiny portion of your team, and the game doesn't rank you on your individual performance. Hence people raging and being fearful of 'elo hell' - which is anything below 1200 rating. It's considered hell because the chances of getting fail team-mates is much higher, since you can be placed with people just getting into ranked matches. And, of course, since any level 30 can join ranked games, irrespective of their 'normal' rating, you can get some matches with total idiots, as they hit you on their way all the way down to ~600 ranked rating
The way I see it, bad players in ranked ( and I mean
bad players) are like rain. If you get high enough you can climb past the clouds, and then you can't get rained on!