"Limits: Since the Flickr API is quite easy to use, it's also quite easy to abuse, which threatens all services relying on the Flickr API. To help prevent this, we limit the access to the API per key. If your application stays under 3600 queries per hour across the whole key (which means the aggregate of all the users of your integration), you'll be fine."
There is also a limit to the number of images you request per call. It is generally 500.