There’s are total of 24 aces in the game. These are special planes that will appear in Free Missions (Normal mode above) after completing specific conditions. They’re worth a good amount of points and they also unlock special camo for their respective plane models once you shot them down. You can trigger their appearance deliberately whenever you replay the missions they’re part off.
To review your collection of Aces, you can check Data Viewer > Campaign > Assault Records. This section of the guide will list all aces you can shot down, the conditions how to spawn them, and other helpful tips to secure your kill. They are listed in the other as they appear in the Assault Records.