How many x's in a text means they like you?

Depends on the context of the relationship and what the person is generally like. The rule of thumb I tend to go by is:

1 x: friends or being polite

2 xs: close friends, or people trying to get closer (generally, if I like someone I'll start using 2 xs but won't go beyond that until I know their feelings, but I'm not a great user of xs at all so the situation might be different for other people)

3 xs or more: they like you/possibly love you

Ultimately, though, I think you need to rely on more than just xs. A lot more.