Oh I believe the test set provided can serve as either a validation or test dataset. I think they exclude the use for a validation dataset just for simplicity in the demo. If you could provide a link or two as reference for when they do this I or someone else could provide a much more direct response.
As far as the second question goes that’s something that’s largely up for debate. I’ve heard a typical split as 80/10/10 for train/validation/test but I’m sure you could find a good resource for the number of data required online.