To answer this, take 3 million and divide by 300 to get the number of visitors per day.
3000000/300= 10000 visitors per day.
Then take the number of visitors per day and divide that by the number of groups of 2 hours there are in an operating Day. From 10-6 is 8 hours, so 4 groups of 2 hours.
Now take 10000 and divide it by the 4 groups to get 2500 people.
There are about 2500 people visiting the museum at any given time.