What is the workflow concurrency limit? It reports an error with just 3 concurrent instances, and there are iteration nodes within the workflow

Discussion
1

When calling the workflow API, the following error occurs:
{"code": "internal_server_error", "message": "The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.", "status": 500}
The concurrency is set to 3. How can this be resolved?