Translation:One needs the majority of the members.
It's more like a proportion than an amount.
There is no measuring stick or measuring cup that has a line saying "majority" or "half" -- those are not really amounts in the way that "two litres" is.
They're relative to a given mass, so (relative) proportions rather than (absolute) amounts.
Compare how you can have "duonlitro da akvo" (half a litre of water) but that "duonlitro" is "duono de litro".