Not like the ones you use, of course.
Most counts I've seen in various places range somewhere in the high end of under ten thousand. Certainly not "tens of thousands."
Not really the point. It's been demonstrated many times that Sadr can raise huge "rent-a-mobs" easily, complete with pre-printed banners and neatly matching flags (paid for by Iranian cash no doubt). This isn't the first such march.
The next step would be Sadr's instructions to his remaining army to rise up again, something that has gone badly for them in the past. We'll see how that works out.