There are 150 embassies with Marines, and only 1000 in the Marine Security Guard MOS which cover that. This works out to an average of less than 7 Marines per facility.
There's nothing magic about that number of 1000 worldwide in the MSG MOS. In a potentially hostile or politically unstable place like North Africa, a platoon of Marines would go a long way.
But 7? Hardly enough to provide 12h shift coverage...
More importantly, how many Marines does it take? One with really good aim and a couple of spare clips.
If bodies start falling back on the crowd, the rest of them will think twice about following the guy who was dumb enough to go first.
And maybe they'll think twice before trying it in the next country.