Historia's are a life-make, where Neinharts makes someone with their former power level I assume, with memories/personality as well. It's only reasonable to assume that a Historia can't have more MP than Neinhart, or the sum of his Historia's =< Neinhart's MP; as the Historia's themselves are created from Neinhart's MP.
It puts into context how much MP a Spriggan has, given he can simultaneously create Hades, Ur, Kyoka, Azuma, Zancrow, Ikaruga, Ezel and Keith... assuming what I said above is correct of course. I've always wondered how the MP works with the demons, given they use anti-magic particles rather than ethernano... this might be a hole in the track of thought that the life makes use the MP from Neinhart.