Instead of just downloading every add-on map ever made for Tremulous, it's usually best to find out what the map rotation is on your favorite server, and just DL any additional maps that you will need to play on that server. If the server has a website or forum, usually you can easily find out what the current rotation is. If the server doesn't have a website or forum, I would recommend playing instead on a server that does.
In addition to the eight default maps, there are probably only about 8-10 add-on maps that you will really need to be able to keep up with the rotation on any given server. A short list would include maps like Gloom2beta2, UTCSb2, ATCSZalpha, Ancient Remains, Hamunaptra, and three to five other, server-specific choices. Check on the servers that you play at regularly.
Cheers.