Last summer, in preparation for going to the Urban Sketching Symposium in Chicago, I tried to practice drawing lots of people. Unfortunately, sometimes they walked away, just as I was going to draw them.
Other times, they were far away to begin with...
It worked best when I just concentrated on people, but often, I only had time for or only had a view of their heads.
After all the practice, once I was in Chicago, I was so awestruck by the architecture, I hardly drew people.