My last period was sept 23 but i dont have a regular how many weeks am i?

Have to guess: For now, because the last menstrual period started 5 weeks ago, we can guess that one's pregnancy is "5 weeks pregnant." Because one's periods were not regular, we actually cannot predict how good our guess is (regular periods lead to accurate predictions). It really doesn't matter, because the typical ultrasound scan at 8-11 weeks will give estimated start and end dates for the pregnancy.