According to tab autocomplete…

$ git
zsh: do you wish to see all 141 possibilities (141 lines)?

But what about the sub options?

$ git clone https://github.com/git/git
$ cd git/builtin
# looking through source, options seem to be declared by OPT
# except for if statements, OPT_END, bug checks, etc.
$ grep -R OPT_ | grep --invert-match --count -E \
"OPT_END|BUG_ON_OPT|if |PARSE_OPT|;$|struct|#define"
1517

Maybe 1500 or so?

edit: Indeed, maybe this number is too low. git show has a huge amount of possibilities on its own, though some may be duplicates and rewords of others.

$ git show --
zsh: do you wish to see all 489 possibilities (163 lines)?
$ man git-show | col -b | grep -E "^       -" --count
98

An attempt at naively parsing the manpages gives a larger number.

$ man $(find /usr/share/man -name "git*") \
| col -b | grep -E "^       -" -c 
1849

Numbers all over the place. I dunno.