Is it normal for a man to wear 18 pair of underwear in 7 days?

That's weird. He's weird. Stop dating weird people.

I would only do such a thing if

1: had 18 pairs of underwear and

2: had copious amounts of sex during that 7 day period.

You sat there and counted his underwear?

Guess it's better than rarely changing.

That's only slightly more than twice a day...does he wear them overnight? I'd consider it normal if he wears a pair at night...reasonable if he just changes during the day.

Three or more times a day would be weird.

Does he workout everyday?

is your significant other male or female

Hasn't he heard of pantyliners?

If you want to reduce your environmental impact you can even get washable ones.

Is he a bigger dude (weight, not dick) an getting the old sweaty fat rash maybe.

Must do a lot of laundry.

Yes that's excessive! That's almost as many condoms as I go through!