https://learn.microsoft.com/en-us/azure/azure-functions/functions-concurrency

*For Python apps, the default HTTP trigger concurrency for all instances sizes is 1.

https://techcommunity.microsoft.com/blog/appsonazureblog/how-to-achieve-high-http-scale-with-azure-functions-flex-consumption/4169736

Not all hosting providers support per-instance concurrency higher than 1, even if some workloads would benefit from it. If your function app doesn’t have compute-intensive operations, per-instance concurrency control may be very helpful. I.e., running four operations concurrently while paying the same is better than paying for one operation at a time.

I haven't been able to try it myself because our service plan doesn't support increasing this numbers.