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.

Note: 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.