By my count, excluding anyone only involved in defending sieges or anyone outside Europe:
9th century: 4
10th century: 8
11th century: 6
12th century: 7
13th century: 4
14th century: 9
15th century: 13
Of course, a decent number of these were disguised as men, and even more didn't actually face combat, just went along on the campaign (Crusades for the most part).
All told, rather few women led armies or units. With how many martial education traits the game gives out to women it'd lead to rather silly results.
