The Swoop! tells of the simultaneous invasion of England by several armies — "England was not merely beneath the heel of the invader. It was beneath the heels of nine invaders. There was barely standing-room." — and features references to many well-known figures of the day, among them the politician Herbert Gladstone, novelist Edgar Wallace, actor-managers Seymour Hicks and George Edwardes, and boxer Bob Fitzsimmons.
The invaders are the Russians under Grand Duke Vodkakoff, the Germans under Prince Otto of Saxe-Pfennig – the reigning British monarch of the day was Edward VII of the House of Saxe-Coburg-Gotha — the Swiss Navy, the Monegasques, a band of Moroccan brigands under Raisuli, the Young Turks, the Mad Mullah from Somalialand, the Chinese under Prince Ping Pong Pang, and the Bollygollans in war canoes.
The initial reaction to the invasion is muted. "It was inevitable, in the height of the Silly Season, that such a topic as the simultaneous invasion of Great Britain by nine foreign powers should be seized upon by the press", but the English are far more interested in cricket and one newspaper placard announces "Surrey Doing Badly" (at cricket), ahead of "German Army Lands in England". And when the Germans begin shelling London — "Fortunately it was August, and there was nobody in town." — the destruction of nearly all the capital's statues, the reduction of the Albert Hall to a heap of picturesque ruins, and the burning of the Royal Academy, earn Prince Otto a hearty vote of thanks from the grateful populace.
The European parties form an alliance and expel the other invaders, but the Swiss soon leave, to be home in time for the winter hotel season, and when Prince Otto and Grand Duke Vodkakoff are offered music hall engagements and the leader of the army of Monaco is not, he takes offence and withdraws his troops.
The two remaining armies are overcome thanks to the stratagems of the indomitable Clarence Chugwater, leader of the Boy Scouts. By causing each commander to become jealous of the other's music hall fees, he succeeds in breaking up the alliance and, in the ensuing chaos, Clarence and his Boy Scouts are able to overcome the invaders.
In The Military Invasion of America, the United States is invaded by armies from Germany, under Prince Otto of Saxe-Pfennig, and Japan, led by General Owoki. Once again it is Clarence Chugwater who saves the day.

Question: How many invaders simultaneously invade England?
Answer: Nine
Alternative answer: Nine.