I found an article from Dec 2014 by a Product Manager at Metalogix about increased sub-site limits for a site collection in O365. The article mentioned that the limit went from 2000 to 100000. This hasn't been updated on the Microsoft site yet and it still says 2000 sub-sites per site collection. I'm really hoping that the Metalogix article is correct and the MS site just hasn't been updated yet! Does anyone know FOR SURE?
Here's a link to the article: