A long-running API request(inbound to Salesforce) is one that takes over 20 seconds. Salesforce allows only 25 long-running to run at a time in an organization.

Concurrent Apex and Concurrent API limits are different.

Check this for Concurrent Apex Limit – https://www.infallibletechie.com/2018/03/what-is-concurrent-request-limit-in.html