I think that the most beautiful photographic mosaics are those made with few tiles, that still are able to reproduce the original image. Mosaics in this category are made with 200 tiles, up to 600 tiles. The main problem is that you need the right tiles, so usually it is necessary to have a very high number of images in the Image Collection. Several thousands, usually I use 20.000 nature photos.
How to create a mosaic with few tiles
Here are some examples: