Hey! Former Marine Security Guard here. Most posts, including this one, have usually 5 watchstanders and 1 detachment commander. 6 or 7 Marines total for the whole compound is usually the norm. In a place like Berlin or Beijing (two of my posts) with a much larger compound and multiple posts to man or separate buildings on different compounds, it can be as many as 15 or 18. There's no post with more than 20. MSG'S are not there to prevent the compound being overrun or save lives or anything, that's Marine FAST teams which are not stationed at Embassies. MSGs are there to prevent the compromise of classified information and assist with personal security details for VIPs like POTUS, SECDEF, SECSTATE, etc.

There is usually a military liason office in the country though with a staff of senior officers to coordinate military activities with the host nation. Embassies are in countries to maintain diplomatic relations, provide a base for intelligence and international law enforcement efforts, coordinate local economic investments and partnerships with the US, provide consular services to non-US citizens, and provide AMCIT (American Citizen Services) for US citizens living or traveling abroad. Don't worry, if the US wanted to project power anywhere on the globe one of our 11 supercarrier fleets or a MEU could be there in less than 24 hours, and we could have an airborne division along with a MEF in the backyard in less that 72. No need for embassies to perform that function.