No one official said anything about a specific number of crewmates. The rumors started with the popular Mugiwara no Ichimi theory. Mugiwara no Ichimi is the japanese word for Straw Hat Pirates, but literally it means something like Straw Hats Gang or something. However it can also mean Straw Hats 13.
When the Vivre Card Databook contained 13 spots for the Straw Hat Pirates, the rumors began to grow. One spot was revealed to be Vivi, and now the other two are revealed to be Merry and Sunny, so nothing new. As if a Databook is going to spoil new Nakama, that would have been strange.