comment-add

comment-add is a variable defined in `newcomment.el'.
Its value is

0

Documentation:
How many more comment chars should be inserted by `comment-region'.
This determines the default value of the numeric argument of `comment-region'.
The `plain' comment style doubles this value.

This should generally stay 0, except for a few modes like Lisp where
it is 1 so that regions are commented with two or three semi-colons.