People are going to show up, that's a fact. Over a million say they're going to do it, but of course the number's going to be drastically lower. That said, even if 99% of them were saying it in jest, that's still 10,000 people that are completely serious. Even if 99.9% of people were being facetious, it's 1,000 people that are guaranteed to show up.
YouTubers are going to capitalize on the event and probably set up fan meetings within a mile or two of the area. Celebs have already claimed they're going to be a part of it, when again in actuality it'll probably be a meet and greet at a local tourist attraction. At the very least there will be more people than usual in Nevada on the day.
I reckon on the day, anywhere between 2 - 5,000 people will show up. There will be warning shots from LMGs and other long-range high-capacity weapons, which is bound to scare off a few of the squeamish ones. Let's say a quarter only just realize the danger they're in and turn around. Now you have 1,500 - 3,750 left to breach the restricted area. At this point they will be forced to kill. A few at the front will be shot at, and I'll assume another quarter run off. That now leaves just 1,125 - 2,800 people to rush their way into the site. Now let's assume there's just 200 personnel and the number of people legitimately trying to storm Area 51 is the lowest estimate, 1,125. They'd only have to kill 5 people each, and it would be a decisive bloodbath against those who are largely unarmed.
For this to even remotely be a success, you'd need at least 50 - 100,000 people. And I don't think that'll happen. Because there's too many variables at play here. Worst case scenario this is the lead-in event for Bluebeam. It's unlikely they'll even find anything, considering it's well-known in /x/ circles that all the interesting shit got moved to Tonopah since too many people knew what Area 51 was. All they'll find are probably some jets and old computers that have long since been wiped clean of anything of worth.