Galveston how far are the offshore rigs

how far are they and how do you know where they are .

Which ones? There are quite a few, some a few miles to some way over 100mi. Maps, chart plotters etc will show them, and you can see them a long way off. A lot more info is needed.

i just want to catch a few snapper,and fish some of the closer rigs for what ever troll for a few kings
within 20 miles

Outside of 9 nauti miles (10.35 statute mile) you cannot keep Red Snapper until the season in federal waters reopens.

Legally that is

my trip is planned for june 19 through 25

Early conversation is that the Red Snapper season will likely be 3 to 10 days opening June 1.

THAT SUCKS THEY NEED TO ISSUE TAGS AND YOU CAN GO WHEN YOU WANT.

Take a trip south to Port Mansfield or South Padre. You can get into red snapper within the 9nm state waters limit there.