20 and it varies for me; while I generally never invert the vertical axis some days I'll play with the horizontal control on normal, and other days inverted. I don't really know why... I think some games I have just default differently so I've gotten use to both choices.
Although my hands love screwing with me - if I'm having trouble playing inverted, I'll go normal just to have my hands react as if I'm still inverted. >.<