The British were aware that the Americans were amassing thousands of men and that they might try to invade Canada and make it a “fourteenth colony.” The British could have sent additional regiments to Canada to turn back the American invasion in the early summer, but this did not occur due to the crisis in Boston and the false hope that thousands of Canadians and Native Americans would fight alongside the British regulars. By the time the American invasion got underway, and the British fully realized their peril, it was impossible to send reinforcements due to the slowness of communications and the worsening weather on the North Atlantic and in the Gulf of Saint-Lawrence.
Carleton knew little about American intentions, but he reasoned there were four routes by which the Americans plausibly could gain entry into Canada. The most likely route, he determined, was via the Richelieu River, which flowed north from American-controlled Lake Champlain in the province of New York. Also possible, though considerably more difficult, was that the Americans would attempt to invade via the Saint-François or Chaudière river valleys, or even along the upper Saint-Lawrence via the western frontier. Carleton therefore placed the greater part of his regulars in the Richelieu valley, kept watch over the other routes, and maintained garrisons in the key towns of Montréal and Québec.
British forces are in red, Americans in blue. Totals include all ranks. The French language is used for places in the province of Québec.
A: Fort Saint-Jean on the Richelieu: 390 men of the 7th and 26th Foot (Major Charles Preston commanding), and 44 men of the Royal Artillery
B: Fort Chambly on the Richelieu: 114 men of the 7th Foot (Major Joseph Stopford commanding), and 5 men of the Royal Artillery
C: Rivière Saint-François: 35 men of the 7th and 26th Foot The placement of this force on the map is quite approximate.
D: Rivière Chaudière: 26 men of the 7th Foot. The placement of this force on the map is quite approximate.
E: Oswegatchie: 28 men of the 8th Foot, and 1 man of the Royal Artillery. This post was on the upper Saint Lawrence, to the west of the area shown on the map. The remainder of the 8th was divided among several posts further west.
F: Lachine: 14 men of the 26th Foot. These men guarded a store of gunpowder and other supplies.
G: Montréal: 110 men of the 26th Foot (plus Lieutenant-Colonel Richard Prescott), and 4 men of the Royal Artillery.
H: Québec (ville): 59 men of the 7th Foot, and 6 men of the Royal Artillery.
In addition to these numbers, 11 of the regulars were on furlough and 69 men of the 26th Foot and 4 men of the Royal Artillery were prisoners of the Americans.
In September, American forces advanced along two of the four possible invasion routes:
A: The American army commanded by Major-General Philip Schuyler and Brigadier-General Richard Montgomery that invaded Canada via the Richelieu.
B: American force commanded by Colonel Benedict Arnold that invaded Canada via the Chaudière (shown here first advancing along the Kennebec River in modern-day Maine).
1. At the time of the Saint-Jean raid, and before Prescott’s arrival, the senior officer was Lieutenant-Colonel Dudley Templer of the 26th; he was no longer with the army at the time of the above dispositions.
I'm working from a number of sources, but the above information on troop totals and location comes primarily from the following document: