I would suggest arranging a meeting with Mendix to go over your requirements as they should be able to give you a detailed response based on your requirements.
As a quick answer, the Mendix Cloud is based on AWS infrastructure, so you will be able to take advantage of that to scale and ensure high availability. There is a high level overview available on the following link.
What is the purpose of the App? Database transactions per session, complexity of microflows, webservice/restservice calls from the app. Aspects that will have impact on performance.
If you want to test the edges of performance you could try the following
1.25 million accesses per minute is an enourmous amount and I have no experience with this on a loadtest with soupui. However, this might be an idea to test it.