It all depends on the program. Usually “-“ is used for ‘short’ options (one-letter, -h), and “–” is used for “long”(er) options (–help).
Short options can usually be combined (so “-h -a” is same as “-ha”)
One exception I found was find which uses single dash, but full names like
find /usr/src -name CVS.
– is not in POSIX, but GNU recommends it and it makes for better readability.