Income can be countable or uncountable depending on the context and use.
The salaries of both partners amounted to a substantial family income
Taking into consideration the incomes of the two brothers, the loan company agreed to give them a mortgage.
But this is beside the point because it's not income that's being compared but the percentage.
When talking about percentages, proportions or money in general, we use much
As much as 50%
As much as half of what they earned
As much as a million dollars
On the other hand, we use many when considering individual units:
As many as 50 parts in 100
As many as a million dollar notes were printed that day.