I am calling this line of code 50000 times in a console application running as webjob in azure:
await cloudBlockBlob.UploadFromFileAsync(sourceFile); // 50 kb files only
Should I care at all about setting ServicePointManager.DefaultConnectionLimit to any other value than the default one?
Can I see somehow or calculate how many connections are used in average for the whole upload to judge wether the default value is fine or I need to increase the value?
Assuming your not using .NET Core yes. On .Net Full framework ServicePointManager.DefaultConnectionLimit is 2 by default. If you want better parallelism set it higher your mileage will vary but start with 20 - 50 and benchmark you should be able to find the optimal value for your situation.
Also bare in mind Azure limits, and handle throttling appropriately.
https://learn.microsoft.com/en-us/azure/azure-subscription-service-limits#storage-limits