Mama’s company had examined the place cards around the big dining-room table, which had enough leaves added so that twenty places could be set.
The passage above comes from Emily's Runaway Imagination by Beverly Cleary.
I'd like to ask a question about the implicit meaning of the highlighted phrase which had enough leaves added. Its literal meaning seems quite obvious, but when it comes to its meaning in context I can't picture what it means. The phrase should be the source of the phrase after so that, that is twenty places could be set. But I don't see any relation between which had enough leaves added and twenty places could be set.