The Condominium Property Act specifies:
“Board of directors
“28 (1) A corporation shall have a board of directors that is to be constituted as provided by the bylaws of the corporation (emphasis added).
“(1.1) At least 2/3 of the membership of the board of directors of a corporation must be unit owners or mortgagees unless the bylaws provide otherwise.”
You should consult your bylaws to see what they say about the size of the board. If your bylaws are more than (say five) years old, consider updating them. There have been many changes to the requirements in the Act and Regulation; your bylaws may be outdated.
As to the size of a board: it depends. My 175-unit condominium has a board of seven. We started with five members nearly 30 years ago. Consider the size of your development and what happens if one board member is away; can you make a good decision if there are only two members present? Do you have a large enough development to recruit five members?