I was not able to find any information about the maximum number of concurrent requests and maximum number of requests in one second to the Visual Recognition service.
Can you provide me with information or link where I can read about limits in general?
There is not a hard limit on concurrency per user. The service is designed to support many users simultaneously, and therefore has capacity to process many requests in parallel. During periods of heavy use however you may occasionally receive a 500 return code, in which case the request should be resubmitted.
Unfortunately there is a legacy error message in the system that tells users they may be submitting too many concurrent requests , but it is very unlikely that the error is actually caused by a users' concurrency. They should be treated like a 500 error code - just resubmit the request.
You should have no problem submitting 30 or 40 requests concurrently.
Standard API keys are limited to 25,000 events per day by default to prevent something like an infinite loop from generating a huge bill. You can have that limit increased by creating a bluemix support ticket. If you really want to do some large scale processing in parallel it would also make sense to get in touch with Support with a bluemix ticket so that we can guide you in the most cost effective and efficient way to use the service.