I don't quite understand what you're asking.
If you want to set it to zero, you leave at default as you have shown.
If you don't want to buy any more than say 5% of the day's trading, just type 5 into the box.
You really need to learn and make a habit of using Analysis project files (.APX) instead of attempts to put every option in AFL. Analysis project contains BOTH formula and ALL settings. It makes your AFL run faster, shorter and clearer and preserves all settings.
This setting can only be changed from UI and it is part of Analysis project. So use Analysis project (.APX).