To me, it’s fairly clear.
Send me an email for every new post (about 2 per day)
This means you’ll get an email for every new post on the forum, and there are about 2 new posts per day.
Maybe it could say:
Send me an email for every new post (there are about 2 new posts per day)
And
Send an email for every new post except my own
just means you’ll get the same as above except your own. I suppose you are expected to know how many posts you create per day.
These seem to be the strings you can change:
js.user.mailing_list_mode.individual_no_echo
Send an email for every new post except my own
js.user.mailing_list_mode.few_per_day
Send me an email for every new post (about 2 per day)
js.user.mailing_list_mode.many_per_day
Send me an email for every new post (about %{dailyEmailEstimate} per day)