An individual RESP, as the name suggests, is only for one child.
A family plan, on the other hand, has no set limit to the number of children who can be added. The rule is that each little scholar in the family RESP must be a close relative—a child, grandchild, or sibling—of the person who opens the RESP. So parents might start one family RESP for their 2 kids, and later add a third child. Or a cool grandma might start a family RESP for her 10 grandchildren who must be siblings.