ATTENTION TO ANYONE USING THIS GUIDE!
I'm like 90% sure that the number of times you reset to retry the Sans date doesn't matter (source: I decompiled the code 馃拃). You can't see the LP bar, but you have to raise his LP to a certain amount in order to get him to like you. Be careful because some options lower his LP!
If you're struggling to get him to like you, I have some hints. With Sans, you have to strike a very delicate balance. Be honest and don't play dumb, but don't directly admit to doing anything cruel. Let him know you like and care about him, but don't be desperate or gross.
Btw, thank you Mar00101 for making the guide! It's very helpful!!