The question you opened
The concert floor and the crowd
A concert floor is 100 m by 60 m and holds about 4 people per square metre.
Roughly how many people are standing on the floor?
about 24,000 people
100 by 60 is 6,000 square metres. Four people on each gives 24,000. Area first, density second, one multiplication each.
A concert floor measuring 100 m by 60 m, packed at about 4 people per square metre, holds roughly 24,000 people. The arithmetic is two steps. The area is 100 times 60, which is 6,000 square metres. Multiply by 4 people per square metre and you get 24,000. Nothing harder than that appears anywhere in the calculation. What makes this an estimation question rather than a multiplication question is that the density is a judgement.
A crowd loose enough to turn around in sits nearer 2 people per square metre. Shoulder to shoulder is nearer 4. Packed until nobody can move is nearer 5. So the honest answer here is a range, roughly 12,000 to 30,000, with 24,000 as the figure that follows from the density you were handed. That is the right way to hold any estimate. Carry the number and the assumption that produced it, so that when someone disagrees you can find out which of you changed the assumption.
The general recipe for crowd sizes is short. Get the area, in metres or in paces if you are standing there. Choose a density from what you can see. Multiply. If the shape is awkward, break it into rectangles and add. The same two step move works far beyond crowds. Books in a library is shelf metres times books per metre. Cars in a car park is area times cars per space.
Cost of a wedding is guests times cost per guest. Split any large unknown into a size and a rate, guess each separately, and the errors in your two guesses will usually partly cancel rather than compound. A last sanity check worth running. Twenty four thousand people is a large arena floor, so if your answer had come out at 200,000 or 2,000, you would know one of the two factors had a wrong number of zeros.
Technique: Area times density