At a minimum it should be one. Maximum probably 5 or something. I would be more for it being just one if Juniors had their signatures removed completely. Then you need ten merit and enough activity to become a Member to get a signature. Either way the requirement should be something and one would be better than nothing.
I quite like the idea of having only members, and above with signatures. That would literally solve so many problems of the bounty spam within the alternate cryptocurrencies section, but also would likely have an impact on several other sections too.