There are a total of 7 Star Cubes in this game. After collecting all of them you need to give them to a NPC, similar to Goku from Dragon Ball Z, in Desert Ruins dungeon. There exact location in a dungeon is not difficult to find out, all you need to do is specifically not miss anything in the dungeon and you are fine by it. Well thats just an excuse of not sharing the location screenshots, LOL.

1. In Thelos' Hotel (Inn). On upper floor near the room where you rest is the star cube.
   
2. In Thelos. In the market square, near the statue with yellow glowing eyes where there is mage guarding in front of huge wooden gate. A star cube will be beside that wooden door.
   
3. In Overworld Inn with baths in it. Ask the innkeeper to rest and the door to your room will open at the end of the inn. Go inside and there will be star cube.
   
4. In Cemetery dungeon.
   
5. In Desert Ruins dungeon.
   
6. In Mountains dungeon.
   
7. In Abandoned Academy dungeon.

I'm not pinpointing the location in the dungeons because if you clear these dungeons completely you will easily find star cubes as these dungeons are pretty much linear. They are not difficult to find in fact they are easy to find and prominent.

If not much I hope it still will be helpful to you guys. If you feel there is anything that needs to be corrected do let me know.