Doctor Who's future beyond the next series has seemingly been confirmed, a article from "Comicon.com" has said
"As part of the BBC’s deal with HBO Max to stream its content in the US, the service will make 11 seasons of the revived [show] available at launch with “an additional three seasons to come."
This means that their will be at least a series 13 and 14 beyond the 12th series due to air in 2020, this news further confirms the long-term future of Doctor Who as the BBC confirmed a partnership with China back in 2017 which included a deal for the show to run till at least Series 15. However it should be stated that this doesn't mean they have been commissioned - just that overseas will have the rights to air the show should they go ahead.
The big question is whether show runner Chris Chibnall will be involved with the further series as reports have suggested that both he and Jodie Whittaker could be departing in the near future - the BBC hasn't confirmed anything but the rumours have stuck around.