I don't know a lot about Napoleon. I heard about this in some movie I was watching on TV today where some guy's like "Napoleon's escaped! He marches on Paris!"
I mean, perhaps he had his army hidden in his sleevy, but this seems unlikely.
As part of the peace settlement after Napoleon's first exile, the great powers made France reduce its armed forces to a fraction of what it was under Napoleon. Thus to start with Napoleon had a huge reserve of former soldiers who felt alienated and disgruntled in the New Post Napoleonic France.
Napoleon was kept informed of the general shaky stance of the restored French kingdom and so he felt that if he allied himself with the anti-Bourbon faction in France and portrayed himself as the revolutionary he once was he could win back the support of France (not that he really ever lost it) and could gamble on one last shot at power.
He set sail from his little island of Elba with 1000 men (he was allowed to keep a small army while ruling his little Kingdom) and landed in France on the 1st of March. The population of France didn't immediately jump over to his side. His first encounter was near the village of Laffrey, in the South of France. A regiment of French soldiers encountered Napoleon's little make shift army. This is where Napoleon famously walked up to them alone and asked:
Soldiers of the 5th, you can shoot your Emperor if you dare! Do you not recognize me as your Emperor? Am I not your old general?
To which the soldiers replied:
Vive l’Empereur
He actually did a bit of lying, claiming the French government had summoned him back and that his return had been approved by the great powers. But none the less the French people were generally happy to see their Emperor again. After this Napoleon walked into the city of Grenoble, where the citizens cheered for him and the administrators got their old Napoleonic uniforms out.
In Paris the Bourbon King Louis XVIII sent another army under Marshall Ney to arrest Napoleon. Ney boasted he would bring Napoleon back in a cage. Of course Ney and the army defected upon seeing Napoleon; but hey, at least he tried.
Napoleon entered Paris on March 20th and after hearing reports that all the allied powers had pledged to form a coalition to overthrow him ordered a full scale mobilization and mass weapons production to begin.
Napoleon could count on the Bourbon army which had largely defected to him. About 200,000 men. Another 100,000 former soldiers quickly joined the army giving Napoleon around 300,000 men. Workshops began pumping out supplies, over a million cartridges a week and over a 1000 uniforms a week.
So within eight weeks Napoleon was able to call upon 300,000 men and very soon he would be able to call upon another 150,000 men when a bunch more men came of military age. In order to defend the frontiers Napoleon had to mobilize National Guard units, which gave him another 170,000; but they would be tied down in garrison duty and were of lesser quality then his regular troops. He also cancelled leave for all soldiers, and began mass enrolling sailors, police, and government officials into the army. This sounds impressive, but the allies would soon be able to put around 800,000 men in the field. The Prussians and British had 200,000 men in the Netherlands alone. So France was still in a rather shaky situation Strategically speaking.
So it was a mixture of the Bourbon army defecting to Napoleon, a large reserve of former soldiers that allowed Napoleon to field such a huge army.
My source for this was "Campaigns of Napoleon" by David Chandler. Also good is the book "Blundering to Glory" by Owen Connelly.
Check out the very well-researched alternative history novel that examines what might have unfolded had Napoleon escaped from St. Helena to New Orleans -- the premise was certainly in the air at the time: http://shannonselin.com/book/napoleon-in-america/