There is really no way to know that, but it is probably a low percentage, since suits are made to work with underclothing, and would be uncomfortable without it.
If male: you dont. If female: dont wear underwear
If trying a bathing suit on, definitely yes. Otherwise it depends on the suit and your level of modesty. Most men's suits have a mesh "undergarment" and women's suits are designed to wear without underwear.
2%
underwear
Some do some dont some dont wear any underwear
No, its fairly unhygenic. I dont suggest it.
It is optional. Some wear and some dont.
Why? Dont wear pants at all.
They only wear their underwear under their suits, it's a jumpsuit
Some men don't wear underwear because they feel it is restricting. There is also concern that the friction and additional heat can reduce sperm count.
u dont
you dont really have a cool type of under wear but personally I dont wear boxers I wear breifs or boxer breifs because it holds it all in... if you know what i mean