Hello @Simon_Persson,
Thank you for your great questions 
What will happen when you hit the limits of your selected cluster? I have a consumer app with a very high number of MAU and want to migrate this to Atlas/Realm. If I pick an M30 instance and this isn’t enough, will it automatically upgrade? For storage, does it also automatically upgrade?
As I mentioned in the post, choosing a cluster tier is an important factor. For production apps, I would recommend M30 or above and you can gradually scale as per your app traffic. Please refer Cluster Auto-Scaling in the documentation.
For storage as well, you can change as needed by your application. Please refer Customize Cluster Storage section for details.
For a global app, is it still fine to chose local deployment and what region should I choose?
Please understand, the deployment model most affects the latency between the SDK and the Realm Cloud Server. For normal requests, global is sometimes better. For Sync, there is a huge reliance on the database so we suggest using a local deployment in the same region as your cluster because this minimizes the latency of the Realm Cloud - MongoDB Connection which is more important than the latency between the device and the Realm Cloud.
I would recommend choosing a cluster region where most of your traffic comes from and then deployment the Realm App in the same region as your cluster.
I hope this answers your queries. Please feel free to ask if there are any more questions.
Cheers, 