Question

Are there any rules for oxidation number of non metals?

Answer 1

You kind of have to memorize it, for me, going from group 14 to 17, The numbers are
-4,-3,-2,-1, those numbers applies to the whole group.

Then as for positive nonmetals, that is a bit tricky, although it contains 4,3,2,1 for each group, some of the elements doesn't have +1, for example fluorine, because fluorine has "highest" electronegativity, it doesn't exist as a positive ion, so there's no +1 for fluorine.

The numbers are going with odd and even numbers for each ions they will exist
Chlorine (Group 17) has -1,1,3,5,7
Bromine (Group 17) has -1,1,3,5
Sulfur (Group 16) has -2,2,4,6
Phosphorus (Group 15) has -3,3,5

Answer 2

Carbon could have every value from -4 to +4. But not ethat they're not net charges. The oxidation number is for our benefit to assist in determining formulas for compounds and for predicting the outcome of redox reactions.