Там ровно ответ на это
> Therefore, HttpClient is intended to be instantiated once and reused throughout the life of an application. Instantiating an HttpClient class for every request will exhaust the number of sockets available under heavy loads. That issue will result in SocketException errors. Possible approaches to solve that problem are based on the creation of the HttpClient object as singleton or static, as explained in this Microsoft article on HttpClient usage. This can be a good solution for short-lived console apps or similar, that run a few times a day.