Ubisoft announced this on their Facebook and Reddit pages.
We are glad to finally share with you that at launch, Rainbow Six Siege will have 11 maps, playable both day or night, all using the full potential of our Real Blast destruction technology. In addition, all post-launch maps will be downloadable for free. We want players to stay with us for the long term and have ambitious post-launch plans that we will detail at a later stage.
someone asked about servers:
Just need to know about dedicated servers?
Dedicated servers for ranked games are confirmed. The dedicated servers are hosted by Ubisoft using the Microsoft Azure platform.
Now I'm sure Ubisoft will charge for stuff in DLC, most likely weapons or upgrades. But at least with all the maps, you should be able to join any game.
Played a lot of Rainbow Six and Rogue Spear so I'm actually looking forward to this one.