This very much depends on the type of images, how diverse and representative of the real world they are, the industry that you are in and acceptable accuracy. We’ve found that somewhere between 1,000 – 10,000 images is a good starting point without committing too much effort. There are several techniques at our disposal that we use to eke as much as we can out of even relatively small image sets and can often get within 60 – 80% of our stated goals. From there we will then put together a specific plan on how to enrich, annotate and augment the data sets to close the gap on the remaining 20%.