Warning: I'm about to quibble. This question is really hard to answer, because there really isn't a stereotypical castle. The answer will vary with time, geography, social and economic circumstances, etc.

First, we need to ask ourselves what a castle is. The answer might seem obvious, but it really isn't. A plausible working definition might be that it is the fortified residence of an aristocrat and his household from which he can dominate the surrounding countryside and seek shelter from attack. But what of all the castles that don't fit this definition? Urban castles, such as the Tower of London, were hardly in a position to control the countryside. Royal castles, such as those that the Anglo-Norman and Angevin kings built in Normandy, or that Edward I built in Wales, might never have a lord or king in residence. These were more or less strategic garrisons which could be sited to control important geographical features (river crossings, roads) or to lock down restive regions. But even this is not so cut and dry as it might seem. John Gillingham argues that Richard I's famous castle of Chateau Gaillard was constructed primarily to act as a base for offensive operations in the Vexin, from which supplies could be forwarded and his supply lines safeguarded, not as a defense - though it would assume that role when the French invaded Normandy in 1203. The size and strength of these fortified places varied widely, from simple timber-and-earth works to massive, multi-layered stone castles. About the most accurate definition I can come up with is that it was a fort, part of a rash of European fort building that occurred from about the late 9th century through the 15th century.

As such, the population of a castle would depend entirely on its individual circumstances. A royal castle in peacetime might possess a small, almost entirely military garrison numbering in the low dozens. In times of war, the garrison of an important royal castle like the aforementioned Chateau Gaillard could swell to the hundreds. The castle of a rural lord would contain not only soldiers, but family, attendants, and servants, the number depending on the wealth and importance of the lord. The closest I can come to giving you an accurate general range is to say that it would be somewhere between the dozens and the hundreds.