It doesn't work like that. All natures have a positive and negative.
Take Brave as an example.
It increases Attack by 10% but decreases Speed by 10%.
Now take Adamant.
It increases Attack by 10% but decreases Special Attack by 10%.
All positive natures increase a specific stat by 10%.
Now take all Attack positive natures.
That's Lonley, Naughty, Adamant and Brave.
They all increase attack by 10% but decrease Defense, Special Defense, Special Attack and Speed respectively. So no attack positive natures increase Attack by more than 10%. This s for all natures though. Sorry if my answer was a bit tough to understand.
Hope I helped!
-It's a secret!