According to Blazor Program Manager Daniel Roth:
We put a Blazor Server app under load with active clients and monitored the latency of the user interactions. In our tests, a single Standard_D1_v2 instance on Azure (1 vCPU, 3.5 GB memory) could handle over 5,000 concurrent users without any degradation in latency. A Standard_D3_V2 instance (4 vCPU, 14GB memory) handled well over 20,000 concurrent clients.
Source: https://devblogs.microsoft.com/dotnet/blazor-server-in-net-core-3-0-scenarios-and-performance/