I'm in early pregnancy I didn't have an ultrasounds yet I found out 6 days ago and I'm experiencing mild back pain and breast tenderness about how many weeks could I be?

When was your last period ?

It will vary a bit from person to person. You could experience those things at 2 weeks, for example, or several weeks later. As Curlyben mentioned, your last period may give you a better idea, but you can always ask about having an ultrasound done. It is pretty standard soon after a pregnancy has been confirmed.