Ontario municipal map with number of members on municipal council per municipality
There are 444 municipalities in Ontario. There is a minimum of five members of council, one of which is the head of council, e.g. Mayor, Reeve, Regional Chair or Warden. Currently, there are 3,306 municipal seats.  Municipal councils are elected every 4 years by the residents of the municipality. The next Ontario municipal election is October 22, 2018. For more information on municipal government see AMO's Municipal 101.  By zooming in to a particular municipality, the resulting flyout shows municipal population, households and the number of members on the municipal council.
Seats on Council

The chart illustrates the number of municipalities with different number of council seats. There is a minimum of five members of council, one of whom is the head of council. 45% or 195 of all 444 municipalities in Ontario have a council size of 5 members.

At Large or Ward
Elections for city councils are held through either a ward or at-large system. Some Ontario municipalities have both, for example, the Village of Merrickville-Wolford is governed by a municipal council composed of 7 duly elected rate-payers. Two members of Council represent the Wolford Ward, two members of Council represent the Merrickville Ward and two members represent both wards as “at large” members.  The Head of Council is elected at large. (Source: 2014 Municipal Elections)
At Large = 58% | Ward = 32% | Ward & At Large = 10%