According to the Brood Master's Larger Brood feature he can have a maximum of 8 small Eidolons at level 13 if he takes the Huge evolution.
I also says that everything apart from the Base Attack and Save bonuses have to be divided among the Eidolons (including natural attacks). Now the problem I have with that is that, A) Each Eidolon base form has at least one natural Attack included already, Biped and Serpentine have 2; B) a level 13 Eidolon can only have 5 Natural attacks and even at level 20 the maximum is 7.
So what happens to those natural attacks form the base form? Does the summoner have to remove them until all eidolons have only 5 natural attacks altogether or do the free evolutions somehow not count against the Maximum? (as far as I recall you can remove your free evolutions like all others but you don't get the EP back)
I think the free attacks do count -- but note that you are not restricted from changing an eidolon's base form in this case because you do not even have a fixed number of eidolons. Thus, at 2nd level, one of your eidolons would have to be a quadruped or aquatic if you don't want to give up any free natural attacks.
At higher levels, the only way to avoid wasting free natural attacks would be to keep your brood down to no more than your maximum attacks could cover -- but then again, if you have a brood of 8 small eidolons, you obviously are not intending them to be combat brutes. In that case, you decide which ones get natural attacks and remove all natural attacks from the rest, making them either weapon users or non-combatants as desired.