Yes, there is a thread limit for both Premium Residential and Private Unmetered Residential proxies.
The thread limit for Premium Residential proxies is 90,000.
The thread limit for Private Unmetered Residential proxies will depend on how many ports you purchase. It will increase by 140 threads for every port that you have access to. The thread limit is across all your ports, however, not per port. You can reference the table below to see the thread limit for each of our standard subscription levels:
You can also see your thread limit using the Proxyrack API. Make a request to "api.proxyrack.net/stats" and you will see your thread limit under "threadLimit".