The limit is between 50 and 60 . In theory, with a factor 50 cream we can be in the sun for 50 times longer than if we do it without protection, with the same result. If you go 10 minutes without burning, with a factor of 20 it will brown for 200 minutes.
It is the doctor who must say which factor is best for us, but also, to achieve the advertised protection we should smear ourselves with 2 mg of cream per cm2 of skin : a bottle would only give 7 times.
