ConsoleApp.cs

using System;
using System.Collections.Generic;
using System.Net.WebSockets;
using System.Threading.Tasks;

namespace ConsoleApp
{
    internal class Program
    {
        public static async Task Main(string[] args)
        {
            var connectionCount = 10000;
            var tasks = new List<ClientWebSocket>(connectionCount);
            for (var i = 0; i < connectionCount; i++)
            {
                var client = new ClientWebSocket();
                client.Options.KeepAliveInterval = TimeSpan.Zero;
                await client.ConnectAsync(new Uri("wss://localhost:5000"), default);
                tasks.Add(client);
            }
            Console.WriteLine("All connected. Press enter to close connections.");
            Console.ReadLine();

            foreach (var s in tasks)
            {
                await s.CloseAsync(WebSocketCloseStatus.NormalClosure, "", default);
            }
        }
    }
}

WebApp.csproj

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFrameworks>net48;net5.0;net6.0</TargetFrameworks>
  </PropertyGroup>

  <ItemGroup Condition="'$(TargetFramework)' == 'net48'">
    <PackageReference Include="Microsoft.Extensions.Hosting" Version="2.1.1" />
    <PackageReference Include="Microsoft.AspNetCore" Version="2.1.7" />
    <PackageReference Include="Microsoft.AspNetCore.Connections.Abstractions" Version="2.1.3" />
    <PackageReference Include="Microsoft.AspNetCore.WebSockets" Version="2.1.7" />
  </ItemGroup>

</Project>

WebAppProgram.cs

using Microsoft.AspNetCore;
using Microsoft.AspNetCore.Connections;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Hosting;

namespace WebApplication7
{
    public class Program
    {
        public static void Main(string[] args)
        {
            WebHost.CreateDefaultBuilder()
                .UseStartup<Startup>()
                .UseKestrel(o =>
                {
                    o.ListenAnyIP(5000, o2 =>
                    {
                        o2.UseHttps();
                    });
                })
                .Build()
                .Run();
        }
    }
}

WebAppStartup.cs

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Connections;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using System;

namespace WebApplication
{
    public class Startup
    {
        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app)
        {
            app.UseWebSockets();
            app.Run(async c =>
            {
                if (!c.WebSockets.IsWebSocketRequest)
                {
                    return;
                }
                var ws = await c.WebSockets.AcceptWebSocketAsync();
#if !NET48
                var buffer = ArraySegment<byte>.Empty;
#else
                var buffer = new ArraySegment<byte>(new byte[0]);
#endif
                await ws.ReceiveAsync(buffer, default);
                await ws.CloseAsync(System.Net.WebSockets.WebSocketCloseStatus.NormalClosure, "", default);
            });
        }
    }
}