This question has actually been asked before here and was answered at length by myself and /u/kenjutsudude.

Check out the link!

http://www.reddit.com/r/AskHistorians/comments/20n7uo/how_common_were_onnabugeisha_female_warriors/

I wrote a massive post on Japanese female warriors in general there.

The short answer is that they were not that common, at least in the sense of a professional female warrior that goes on military campaigns and fights as a front line soldier.

Female soldiers would have generally been seen as a sign of desperation and the assumption would be that they were fighting to defend their homes against raiders, bandits, invaders, etc. while the regular standing forces were busy fighting a war elsewhere, incapacitated, etc.

Professional female warriors have always been exceptional in Japan, not standard.

As for the Sengoku Period specifically, it would have been even tougher for a woman to make headway during the time period because of the nature of warfare had changed. It was all about massed armies and total warfare, with much of combat now revolving around firearms and massed spears.

Even if a female warrior was very skilled in martial arts, she would have a hard time making a name for herself, which she definitely would need to be even moderately accepted by her peers, who would look down on her because she was a woman.