Not a service member/vet, but I do have a lot of knowledge about this.
Let's say a raid attempt DOES happen. Area 51 is HEAVILY protected, and there will be no chance of people raiding it. I'm not sure exactly what kind of protection it has, but I imagine there to be sniper rifles, machine guns that can fire at 3,000+ rounds per minute, and missiles that are capable of wiping out the giant crowd of people near the very beginning of the raid.
However, I do not think thousands of people will show up. Maybe a few hundred people will show up and then just give up when they see that there aren't 300K people.