Season 5 also had 5 (Bowling Moms, Christie, Kim and Nicole). The Family Edition had an even higher proportion (11 out of 16), but that race never happened.
Come to think of it, Season 5 had 9 females in the final 12 racers (Mirna and Schmirna and the twins). Of course, it would be hard not to have a bunch of women, since there were only 9 men total out of 11 teams.