More tile images you have in the library, the better mosaic you'll get.
If you don't mind to use a heavy color correction you may start with 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 quite a nice work.