I am currently about to setup a cactus garden bed in my front yard. As I am putting a link block retaining edge on it I have to fill the level about 20cm. I need to get a few cubic meters of soil so what would be the best ratio and type of soil/sand/gravel to mix into the bed. My current soil is a loamy soil so I need to get better drainage. Also what size gravel do most people use for the top layer I was looking at somewhere around the 10mm size.

Sandy/mineral loam is great for cacti, and drains quite freely ime, just make sure its not made up of large amounts of clay and organic matter.

Having your bed raised like you have indicated is probably more important than the soil mix itself.

If your worried about drainage though, maybe put a layer of gravel in the base of your raised bed.

I like yellow limestone gravel as a top layer, the size doesn't matter much, just get what looks good