Chicago 1 and 2 are the most used and most important servers. Dallas 1 and 2 are OK as well (but I'm biased towards Chicago).
The two #3 servers are hot garbage that allow dumb weapons and sniper. I promise that the only reason those are sometimes used is not because people want to practice that trash. It's because people just want to dm and they join whatever server has people in it. Either the initial 1-2 people in there are the jokers that really feel like they need to practice sniper/dumb weapons in dm or people literally accidentally happened to click on those servers when all six were empty and then people are going to continue to join what's populated until prime time dies out.
The maximum number of servers needed to be up is 3 (4 if you really want) and a safe minimum is 2. It is super rare to see 3 servers filled up.
Also please if we could fix the issue that causes you to download gigs of sound files unless you have the "cl_downloadfilter mapsonly" cvar that would be great. My understanding is that other servers that are actually maintained have had this fixed for some time now. As it is now it's hurting the population of the servers. Either the server says 8/8 while actually 1-2 people aren't playing but downloading maps for 15 minutes or plenty of people probably just give up and don't play.
Also the afk kickers are super inconsistent between the servers. The bad one(s) are where the kicker seems to not work at all and someone just afks in spec forever.