Much of it depends on the band. The best bands appear on Market Nights, Tue at SS, Wed at LSL and Thur at BW. Also Friday and Saturday tend to have better bands. There are about 400 chairs at each square plus at Brownwood you have the bleachers.
Because of the bleachers, at Brownwood you would only need to bring your own chair on a Friday or Saturday if Rocky & the Rollers, Johnny Wild, the Hooligans or the Spazmatics were playing. Most of the other bands draw less, Cactus Jack and Uncle Bob will get decent crowds.
Same could be said for LSL and SS about those bands, also it depends on the time you arrive. If you arrive at 4:30 to 4:45 before the bands start, you can have just about any spot you want. If you arrive about 6:30 or a little later, you'll find empty seats available since many people will leave to go eat after the first set. It's mainly if you arrive at the peak time which is a little after 5pm and until the first set is over, about 6pm to 6:30.
Here's a link to the webcams, take a look about 5pm, 6pm, 7pm at each square and you'll get the idea. If you're on a LSV you'll get a newspaper delivered free to your driveway, the entertainment will be listed there. Also there are online sources to find what band is playing where.
The Villages Squares Webcams