to find out how many weeks of pregnancy

hi all, i am new becoming as mum. i havent went to doc to check about my pregnancy yet to check how many weeks already. somehow my last period is on 15 dec 2019 and i had sex intercourse on 21 dec, about 27 dec i went to see doc to have someblood test n the result it was negative that i am not pregnant. lets say i had intercouse pretty active since 21 dec 2019 till jan 14 2020. by right 12 jan my period should be come n yet it didnt, so i went to check on my own with pregnancy kits n turn out still negative. on 13 jan morning early i did try to check again n still negative. after a week which is 19 of jan 2020 i try to check n finally i am positive, so my quest is how do i count to knows how many weeks i am pregnant now as i till up today?