No it's done via raffle. 12 of the 18 will deliberate but those 12 are randomly selected from the pool.
Yup, I just found the process:
A pool of 150 prospective jurors was summoned to Kenosha County court Monday for the beginning of jury selection. Cafferty says the judge is responsible for narrowing that group down to 34 people.
“Once they get it down to 34, each side will get seven strikes and they'll bring it down to 20 which will be the group that will actually hear the case,” he explained.
Once the jury pool is down to 20, the trial will begin. Cafferty says eight of those jurors will not find out they are alternates until all of the evidence has been presented.
"I think this judge literally has this old-school bingo looking device and he'll put the numbers into the device, his clerk will spin it and she'll start to pick out numbers randomly, and she'll say ‘juror number 127, you're excused, juror number 112, you're excused,’ and they'll be left with twelve,” Cafferty explained.
IN-DEPTH: How jury selection works in Rittenhouse trial (tmj4.com)