Hi,
There's no documentation of the maximum network throughput for the B-series Azure VMs, this has been raised also over here:
https://github.com/MicrosoftDocs/azure-docs/issues/8765
You can simply test the network throughput by following the official Microsoft guideline over here:
Bandwidth/Throughput testing (NTTTCP)
https://learn.microsoft.com/en-us/azure/virtual-network/virtual-network-bandwidth-testing
You can also optimize the network throughput for an Azure VM:
Optimize network throughput for Azure virtual machines
https://learn.microsoft.com/en-us/azure/virtual-network/virtual-network-optimize-network-bandwidth
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Best regards,
Leon