The global "tags" can be iterated upon for every unique tag which is set in the
level. If a tag is set on a sector/line/thing, it will be included. Taking the
length of "tags" will give you the number of these unique tags. (If a tag is
set on multiple sectors/lines/things, it will only be counted once though.)

For sectors, lines and mapthings, call the field "tagged". This function takes
one argument, which is the tag. The return value can be iterated over for all
the sectors/lines/things with that tag. The length can also be taken for the
number of such objects. If no argument is given, the global tag is default.