More tile images you have in library, better mosaic you'll get.
If you don't mind to use a heavy color correction you may use about 100 or 200 images.
If you want to make a better mosaic with less color correction from the same limited number of tiles you
should cut and crop these images as said here. It will give you
500 - 1000 smaller tiles that usually is
enough for a quite nice work.
To render mosaic with default settings - Repeat = <Auto> and Distance=<6> you had to have at least
144 different tiles in your library. If you have less - you'll get <Not enough tiles> error message and in
that case you had to either add tiles into library either decrease the Distance setting.
Here are few more reference numbers:
Distance: 1 - requires 4 images in library
Distance: 2 - requires 16 images in library
Distance: 3 - requires 36 images in library
Distance: 4 - requires 64 images in library
Distance: 5 - requires 100 images in library
Distance: 6 - requires 144 images in library
Or you may use this formula:
Distance: N - requires N*N*4 images in library