Need a Regex that contains at least one number, zero or more letters, no spaces, min/max

This is a typical password validation with your requirements.
Note that this will also match 8-13 digits as well (but it is requested).

Ten million + 1 (and counting) happy customers ..

^(?=.*\d)[a-zA-Z\d]{8,13}$

Explained

 ^                     # Beginning of string
 (?= .* \d )           # Lookahead for a digit
 [a-zA-Z\d]{8,13}      # Consume 8 to 13 alphanum characters
 $                     # End of string