That's not the paper, that's just the abstract, which doesn't actually talk about sample size. It does point out an important limitation of the research: it looks at number of children specifically at 23 and 24 years old, rather than the full range of ages at which people have children in Canada. Which is the other limitation: it only looked at Canada, and the abstract doesn't say which parts of Canada, so there's so much information missing that it's impossible to tell what this study actually found.
