The one in Gaitherburg is always crowded. I went there last time and decided to go up to Germantown instead. That was much better.
It's hard to tell. Almost everyone I know got an absentee ballot. What I've heard on the news is those who requested absentee tend to be upper middle class white people, and lower income people of color were less likely to do that. So areas of the county that have larger lower income populations might have a lot of people at the polls.
The one person I know who will vote in person works right next to the Executive Office Building in Rockville, and plans to vote there. She can keep an eye on lines throughout the day and go when it's not busy.
The County Board of Elections had a real-time counter of crowds at each site during the last election. I assume they will have that again, and you can check on line.