What are the chances of becoming pregnant from pre-cum. Percentage, how many in ever # of women.?

See below... If you partner partially ejaculated in your vagina, and you are not on any form of protection, then the chance of pregnancy might still be as high as 20%. You need to check a pregnancy test about two weeks after this occurrence.
Don't know % from. pre-ejaculate. Most studies have shown low levels of sperm or no sperm in pre-ejaculate with sperm not being motile (however, don't know how promptly samples were examined). One small study published in 2011 showed that there are some individuals with a high motile sperm count in pre-ejaculate. See: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3564677/