You need to review your articles of incorporation and your bylaws to see what they say about the number of individuals who can serve on your Board and how they are supposed to be selected. Those governing documents may give you the procedure. If you don’t have bylaws (or can’t find them), consult an attorney to help you draft an appropriate set. If you have access to a state association of nonprofits or an academic nonprofit management support center, you might want to attend some programs on corporate governance to get a broader overview of the issues as you grow the organization.