Heather O. answered 03/14/19
Experienced SAT, French, math, and humanities tutor
Even though there may seem to be an overwhelmingly large population of people to count, people are always countable--so you would always use "many" rather than "much" when referring to people.
Countability can be tricky if you think about it as a process for which you would be responsible. But remember that even grains of sand on a beach could, theoretically, be counted because each grain is an individual unit. So even though there are more grains than one person could ever count, there are still *many* grains of sand on the beach, not *much* grains of sand on the beach.