do I have to reset my router to factory defaults before flashing ( TP-LINK Archer C50 v1.0) ?
Generally speaking; No, it is not necessary.
if you run the stock firmware and you want to flash OpenWrt, it is pretty straight forward. Settings will not be carried over.
If you want to upgrade from OpenWrt to a newer version, then you have the option to keep settings. However this must be used with care and it is recommended not to use it over major release upgrades (e.g from 17.01.7 to 18.06.8, or from 18.06.8 to 19.07.2)
Thank you trendy
so, I just need to flash the router ( it is setup as AP but I suppose that does not matter?)
No it doesn't.