You should focus on only one or two games. I wouldn't try and focus on lots of games at the moment for two reasons..

1) Your subscribers may not like the other games you are uploading, and only like one. (EG: Some CoD fans may not like Minecraft, and vise versa)

2) It will become difficult to focus on lots of games!

If I were you, I'd choose ONE game. I think you should focus on Minecraft first, as Minecraft videos are alot quicker and easier to make than CoD montages and Xbox games. When you start getting big on your Minecraft channel, start a new channel with a similar name but with the game you are focusing on in it. For example, if your Minecraft channel name is PlayMinecraft, have the other channel called PlayCoD. You can then link that CoD Channel on your Minecraft channel, which will mean that your subscribers will have the option whether they want to see CoD videos.

It doesn't have to be CoD by the way, it can be any xbox game. I was just using it as an example.

Hope this helped you.