I'm not sure exactly what you mean.  Do you mean that you only see one map in the Map List when after you click on <Single Player> from the main menu?

I just loaded up the game and all I need to do is click <Single Player> and then choose a map to play from the menu.  (I think there are about 10-15 small maps to choose from.)  You can change the map size category and see other maps by clicking on <Change Map> at the bottom of the map list window.

If you can really only choose from a single map then you might have some sort of an issue with your installation.  If so you could send an email to [email protected] or maybe read through older threads in the Technical forum.

Find the galaxy folder in your C:\Program Files\Stardock Games\Sins of a Solar Empire folder. If you have plenty of maps in that folder, it is probably your galaxy.manifest file that is not working correctly.

I'm not sure if the game generates a new galaxy.manifest file or not. I was going to suggest trying Harpo's galaxy manifest generator, but if it does indeed do that it could work.

If you are playing on a map with more than one solar system, you need to research a long range jumps (different name depending on the race) located in the second or third level of the civilian technology tree. After that, you can jump from the star in your home system to the star in any other solar system.