Is this really an issue? I mean, we've discussed the situation hypothetically, but I can't imagine any scenario in which a mutant with more than 1 extra arm would realistically be able to be purchased and be a threat.
For the warband to spend enough to recruit a mutant with, say, a total of 6 arms, they would need 305 gold, plus whatever you're equipping him with. All that just for a model with a standard statline plust a ton of attacks. That means one of two things. Either you're crippling yourself at the beginning of the campaign with very limited numbers, and therefore the crazy guy with 5 attacks really isn't that scary because he's only backed up by a magister and maybe a couple of brethren or something (not to mention that you'll be making money very slowly, because you probably won't have enough gold initially to recruit many heroes). If you didn't hire him from the get-go, then you're talking about hording your gold for a while, thus not replacing lost henchmen and equipment, and also limiting your income by leaving one mutant off the roster until you amassed the gold to hire him, so once again he's being brought into a warband that's already had some set-backs, and now against more experienced opponents.
Maybe the 6 arms is a bit of an extreme example, so let's say you went with 2 extra arms, thus 4 arms in total. Now you're looking at a guy that costs 145 gold before equipment because he has 4 arms, meaning potentially 4 attacks or some mix and match to get him a good armor save. To me, more than 1 extra arm is just a huge gold sink for not a whole lot of benefit. When I have three attacks already, is the fourth really worth 80 gold? At a certain point, one additional attack on top of many isn't really that much of a benefit.
While a many-armed mutant is hypothetically very threatening, to obtain such a character seems completely impractical to me.