A single field army or a state's total number of soldiers? The Ming dynasty in China (1388-1644) mobilized on occasion a million men, out of a total population 160-200 million, but would not have concentrated them into a single army.

I don't know how to answer this question totally because I don't know enough about the entire world's armies through history. I can provide some insight regarding 19th century US and European armies. The US/Union Army might have topped out at a million active duty personnel for a few months from March to May 1865; though, they were never all present at once place and many were non-combatant roles.

Prior to this, the closest I am aware of would probably be Napoleon's Grand Armee in 1812 which had about 650,000 actual soldiers. If you combined the forces of the Sixth Coalition opposing Napoleon around 1814, they would have had over a million soldiers, but I'm not sure if that really answers your question since they too were not all in once place, and were a multi-nation coalition.