90 seats.

"Once you add up the upcoming special elections, the open-seat races, and the races where there’s some indicator of trouble for a Democratic opponent — a particularly strong challenger, favorable district demographics, surprising fundraising numbers, a particularly weak Democratic incumbent, or a combination of these factors — you come up with more than 90 House races. If the GOP wins only half the seats listed below, they win back the House."

http://article.nationalreview.com/433360/house-races-in-2010/jim-geraghty