If i had sex with #1 on december 20, 2022. got my period on december 24,2022. had sex with #2 on january 7, 12th and so on . found out i was pregnant january 19 . and at my first ultrasound ob on february 21 did not change my lmp which is father?

I do not understand your last sentence. Based on the history, regardless of the fetal age by ultrasound, then if you truly had had your period on 12/24/2022, i.e. a 3-5 day vaginal bleeding with typical flow, then the conception had to be after that date then your second partner has to be the father. If you have doubt, just have a DNA test later. Take care.