What is the correct abbreviation for the word "numbers"?

It is highly unlikely that there is a global standard. It differs based on practice and the standards set by the relevant authorities (publishers and the like).

Ex: The Oxford Journal Instructions for Authors suggests (Sec. 2.3):

Abbreviations where the last letter of the singular word is not included take a full stop (vol., vols./ed., eds.). The abbreviation for number is no./nos. Abbreviated unit of measurements do not take a full stop (lb, mm, kg) and do not take a final 's' in the plural.

This is a suggestion from Cambridge Dictionary for use of no. as the abbreviation for number.

Cambridge dictionary only gives the abbreviation no. when number refers to a numeral, a position or identification, not a quantity, amount or calculation : http://dictionary.cambridge.org/dictionary/british/number_1

Therefore I understand "Number of guests" should not be abbreviated.

In other words, the ordinal number (e.g. No. 3, which indicates a position in an ordered list) is abbreviated, but the cardinal number (e.g. Number of guests, which indicates a numeric quantity) is not abbreviated.

I use "num" or "nm" instead of "no" to avoid confusion with "yes/no" meaning -- especially in my computer programming variable names.