The number of maps you have access to is based off of your level (this changed with the 2.0 update recently). Each map has a certain minimum level the player must be in order to have access to it. If you want access to additional maps, simply level up! After you level you'll receive a big pop-up screen congratulating you on reaching a new level and it will display for you exactly which maps you unlock. Win a few games and get some achievements and you'll get through the early levels fairly quick, where the majority of the map unlocks are.
Edit: Here's a link that shows you basic unlocks up to level 30 that might be helpful. http://wiki.warlight.net/index.php/Levels
Edited 10/12/2013 15:27:09