How fast does pre cum die and does it guaranatee pregnancy? how much sperm is in it

Varies.: The fact is that secretions from the penis prior to ejaculation do contain sperm. Inside the body, sperm can live for several days. Outside, it's likely a matter of hours before they perish. You can start with a home pregnancy test first if you are concerned about pregnancy. Pre-ejaculate secretions contain less sperm than an ejaculate contains, but pregnancy is possible with exposure to it.

No guarantee: 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/