Very interesting question! The reason we use 'much' is because it's not actually applying the 'much' to 'senators.' You're correct that it would
always be wrong to write 'much senators' rather than 'many senators'.
However, in this sentence, 'however much' is part of an idiom. It works like this:
'However much (sentence 1), (sentence 2)'.
The meaning is that doing what's described in sentence 1
should prevent sentence 2 from happening, but it doesn't. For instance:
'However much she studies, she always gets bad grades.' = Studying should prevent bad grades, but she gets them anyways.
'However much teenagers eat, they rarely gain weight.' = Eating a lot should prevent 'rarely gaining weight' (that is, it should cause you to gain weight), but it doesn't.
'However much Maltania's senators agree, the bill will not pass.' = Their agreement should prevent the bill from not passing, but it won't.