We normally expect adjectives to have a noun that they refer to; however, some adjectives get their noun from the context, and we are expected to supply it ourselves. I recommend that you supply the noun anyway, particularly when you write expositorily (when you’re explaining something). In English, using “many” without its noun assumes you’re referring to people. Sentences such as these are taken to refer to people:
How many died in the hurricane?
How many came to your party?
But we use the noun when we’re not referring to people:
So, pardner, how many head of cattle yeh got out back?
How many pages in your term paper?
So I suppose these guys have a point.
So put in the noun!