As an estimate, an average knight's fee, or manor, might have had a population of about 300. So, it might have taken 300 people to support a knight. (Though this probably varied across Europe and across time. This estimate is based on England in about 1100.)

To calculate this:

Population of England in 1100 = 2.0m

http://www.localhistories.org/population.html (estimated population of England at the time of the Domesday Book, 1086)

Number of Knight's fees (or manors) in England = 5,000 - 6,000

http://www.buildinghistory.org/manors.shtml

90% of the population lived in the countryside, rather than in towns.

http://faculty.history.wisc.edu/sommerville/123/123%2013%20society.htm

Rural population of England = 1.8m

1.8m population / 5,000manors = 360 population per manor

1.8m population / 6,000 manors = 300 population per manor

(Not all manors supported knights but all manors were supposed to be about the size to be able to support a knight).

So, about a population of 300 people needed to support a knight.

(Note: That is an estimate of the total population of the manor, including the knight, his family, retainers and servants, clergy, craftsmen, men, women and children; not strictly an estimate of the number of 'peasants', though peasants would have been the majority.)