As a rough estimate, a streetcar line is more efficient than a bus line above 40,000 passengers per day. My city (Dresden, Germany) has a bus line with 70,000 passengers per day, and you can really see it cracking at the seams. During rush hour, busses run every 3 minutes, and that interval cannot be increased any further because the busses would start blocking each other, thus backing up from the bus stop into the road and creating additional congestion.

The only thing the city can do (and that's what they're working on) is to upgrade from bus to streetcar. Dresden has some of the longest trams in the world, at 45 meters (50 yards), and these carry 3-4 bus loads at once.