Trump better nip this in the bud...those sonsofbitches will keep doing this just so there are empty seats.
There is nothing that he can do about it, other than counter the disinformation.
The way that it works is that the local Fire Marshall sets a hard limit for how many will be allowed inside the venue for the event. Last evening it was 11,500 apparently. When you have close to 2,000 of the people admitted choosing to stand on the floor to be closer to the stage, you are going to end up with "empty seats."
The Fire Marshalls in most locations will not up the total number admitted to fill-in the empty seats with more people. The campaign always make the request before the event starts, and it is usually refused. IIRC, in 2016 they were given permission to let in a 200-300 more, on three occasions.
And at the 2015-2016 Trump rallies that we attended, no one asked for our tickets before going into the the venues. It is was first in the lines, first allowed in (up to the FM declared limit). So the plan that they've tried in the past as well: let's get a bunch of tickets and not show up, doesn't work at all, since you don't need a ticket to enter....
.