9 home games, in that one of the Chargers-Raiders games already counts as a home game. While there may be an advantage in having a little less travelling, this may be off-set in that in both of the games with the stadium mate, one would effectively not have a home field advantage.
There are 16 regular season games in the season, 8 at home and 8 on the road. For this situation both teams combined would play 16 games in that stadium.