What is HttpClient's default maximum connections

The answer is not complete. It depends on implementation. In .net core ServicePointManager.DefaultConnectionLimit setting is not used, HttpClientHandler.MaxConnectionsPerServer should be used instead.

https://blogs.msdn.microsoft.com/timomta/2017/10/23/controlling-the-number-of-outgoing-connections-from-httpclient-net-core-or-full-framework/

It uses the same ServicePointManager so the answer is yes.

You can change the limit programmatically though if you want, see this