My last period was jan 9th. it ended on the 12th (regular 28 day sched) i had sex with guy a jan 14th. then i had sex with guy b jan 20th. due oct 17?

Pregnancy: The only way to know for certain who the biological father is would be to ask them to consent to a paternity test.

Guy B: Guy B was most likely the father. Intercourse with Guy A was on Day 6 of your cycle. Intercourse with Guy B was on day 12. Day 12 is very close to when you would most likely ovulate. However, the only way to really be sure is to do a paternity test after birth. Frequently the Baby's blood type will tell you who dad is, if you know Guy A&B's blood types. Sometimes more testing is needed.

Paternity test: If you have a normal 28 day cycle and due date is based on an early ultrasound then sex for conception likely occurred between 1/19 and 1/24. There are a lot of assumptions in that answer so I would recommend having a definitive paternity test after your baby is born.

Could be either one: I hope you like them both. Guy B is more likely but you will not know until a DNA test.