People can post from anywhere, but need to be physically present to show up to a parade. And it's easy for a single person to post multiple times. FWIW apparently the weather sucked too.

Weirdly, I haven't seen news outlets provide estimates of the number of attendees. The closest I've seen is

attendance appeared to fall far short of early predictions that as many as 200,000 people would attend

from CBC. It sounds like it was low turnout, but I'm not clear how low.

Assuming the photos are legit, the No Kings protests clearly got a lot of people out.