If I double the number of speakers how much does the volume double by?

Yes supposedly only increases by 3db but thats gotta be a average. Ive done several competitions and one paticular time i was looking for a little more umph so i decided to add another sub but i changed the box setup for that one sub. I had 4 in a sealed chamber and in 1 corner i ran a single sub in a larger ported design. Long story short before i added the sub i hit a 138.6db and after the sub a 143.7db. Thats closer to a 5db increase. Of course i added another amp and changed box design but my point is it can vary in increase but 3db increase at least.