Your server cycles through the list of maps in the order they are shown in mapcycle.txt. If you have 50 maps in your ns/maps folder your server will only select those in mapcycle.txt
Note: if you don't have adminmod or amxmod, the map cycling works funky. It will usually run the maps in the same order, every single time.
When I play specialists on lan, I get to pick the first map when I start the server. After that, it starts at the top of the map list and goes straight through them. If you want to change the pattern, you'll have to start the server again. This applies to NS too, but I don't have enough people around to play it on lan <!--emo&:p--><img src='http://www.unknownworlds.com/forums/html/emoticons/tounge.gif' border='0' style='vertical-align:middle' alt='tounge.gif'><!--endemo-->
If you're going to do lan things often, it's worthwhile to get an admin mod. Sadly, I'm lazy, and haven't done it myself yet.