Bengals rookie safety, Corey Lynch, went on Injured Reserve today after suffering a "bone injury" on special teams against the Houston Texans. Lynch finishes the season with three tackles, a sack and an interception on defense in seven game played. He also had five special teams stops.
Currently, the Bengals have 10 players on injured reserve with Lynch's addition -- three rookies from the 2008 NFL draft (Keith Rivers and Matt Sherry being the others). However, Ethan Kilmer (thigh injury) was placed on waived/injury before the season started and Maxwell was released from the team (mid-October) after suffering a season-ending injury during preseason. Both players were initially placed on IR before being waived with an injury settlement (though it feels like I'm missing someone else).
So that number is actually 12, though only 10 remain on the team's roster.