Here is the sentence :
人数が多ければそれだけお前達が有利になる。
I've difficulties finding information about this word. From the examples I read, it seems to me that, when used after a clause ending in ば its meaning is similar to the ば...ほど construction :
人数が多ければ多いほど前達が有利になる。
The more people there will be, the more advantage you will have.
Is this what それだけ means in this case? And if it is, does it have the same meaning in other constructions?