According to my grammar book we should use "many more + something" with count nouns and "much more + something" with uncount ones. For example:
1) In St.Petersburg there are many more museums than in Moscow
2) There is much more milk in this plate than in the one on the table
Can we use "much" instead of "many" in sentence #1?
I know where you're coming from. In Russian both "many" and "much" would simply mean "much" in the context you've provided. But, as we can see, in English it does depend on whether the noun is countable.