How likely does pre ejaculation fluid can impregnate a woman if a couple just had sex once? and how likely does the said fluid contain sperm?

Pre-cum can result in pregnancy. How often is not feasible to state and is irrelevant. Full ejacution in the vagina does not result in pregnancy every time. If you do not wish to be pregnant, use contraception all the time, every time. Safe sex: https://www.hopkinsmedicine.org/health/wellness-and-prevention/safer-sex-guidelines

All of the semen (fluid) emerging from an erect penis will contain some sperm. Not at the higher concentration of the main ejaculate, but some. It only takes one sperm making the trip into the cervix & beyond to trigger pregnancy. The chance is random, as is any unprotected sex. Either it will or not. It is less than with full ejaculate but not zero. The "pull out" method is not BC.