RESTlet concurrency request limit
There is a maximum number of incoming https requests (the account concurrency limit) that your account can handle at the same time. Exceeding that limit results in Errors Related to Concurrency Violations. You can see what the limit is, along with some stats assoicated with integrations, at Integration Governance.
You will need to understand how Web Services and RESTlet Concurrency Governance works to understand how the account concurrency limit is calculated and the ways to increase it.