@konradpsiuk, MS Azure provides a robust infrastructure for the purpose. I would suggest using Azure's well-Architected framework with the sample reference Architecture that can be found here. Answering your question, IoT Hub is your entrance point to the cloud (i.e., Cloud Gateway) and acts as a message broker ingesting the messages from the IoT devices (sensors). The number of partitions can be set from the very beginning (when you provision the Hub) and can't be changed later. They're used to eliminate the contentions when reading the messages and writing them to the event streams. Most of the Hubs would only need 4 partitions. However, to be certain, I'd recommend you to skim through the following topic: How many partitions do I need?
With regard to scalability, consider the IoT Hub quotas, throttling, and limitations here. The free tier only allows 8000 messages a day (and simple math shows that it's not even enough for 1 sensor and your intent to send a message every 10 seconds). It's good for the testing purposes though (but you have to be aware of the message size limit there, which is 0.5Kb). From the scalability perspective, the IoT Hub can easily be scaled just by moving to the next consumption tier (B or S).
The best practice suggests you would need a distinct infrastructure for the DEV, TEST (STAGING) nad Production, although, you don't need to provision everything at once from the very beginning. Standup your DEV environment first and make sure you're comfortable with it.
Let me know if you have any further questions.
ps. If I answered your question, please don't forget to mark it as answer/like it.
