MQTT for 50 000 devices

We are at the very early stages of developing an IoT solution. This will be something internal within the company, and currently we are in the phase of looking into the capabilities of the technologies.

The idea is that we will have some devices taking measurements and sending to the cloud. The final product will be for 50,000 devices, each sending a measurement every 30 seconds. Simple measurements (just a real number with a timestamp).

• Do you have experience using MQTT for this size of systems? I've seen posts claiming up to a million devices, but something more concrete would be nice. I've only used MQTT over ZigBee for the 20 devices in my house... so, not much experience.
• Any advice on the database to be used?
• Suggestions on MQTT broker software? It should be self-hosted (eventually) due to data-privacy issues. Price is not really a problem.
• Anything else we should make sure to look into.

Just to make clear, we will probably hire experts to do this in the end if the project goes forward. It's a large project that is important to the company. However, I like to read up and be prepared.