Depends on how you define watching...
Intelligence agencies are watching billions of people, but do they watch you? Maybe.
Bruce Dickinson on an Iron Maiden concert is watching thousands of people, but is he watching you? Perhaps.

No, You can give but a passing glance to all or an intense focus on but a few.

I would never try to watch that many people, too big of a crowd, and I detest large crowds.
Large crowds make me think of cattle, and they act much the same way.