How many DPO

Tia • Been with my hubby for just over 6 years. Married on 10/11/13. Prego with babe # 1!!!
Ok so I've look everywhere online and cannot find a solidified answer. My OPK said I had my PEAK day on January 19th. Does that mean I ovulated on the 20th or the 21st? How exactly are you supposed to know your exact ovulation date? I'm having issues determining how many DPO I am. Any ideas? I've heard 24-48 hours after your peak. So am I just supposed to guess how many DPO I am?!