How far out can 8 week scan be?

Please clarify your question.

I guess if you wait a week it is then a 9 week scan

On the 1st of July scan came back 8week 2 days but had sex with ex at 10weeks 3 days and 7weeks 1 days with me who's the dad ?

She had sex with ex on 19th April (10weeks 3days) then with me on 12th may(7weeks 1day) on 1st July scan came back 8weeks 2days who the dad ?

You won't know who the father is until a DNA test is done.

There is no way to be exact on when conception happened. Due dates are estimated at best and are no exact.

So you will have to get a DNA test done to determine who the father is