tile images you have in the library, the better mosaic you'll get.
you don't mind to use a heavy color correction you may start with about
100 or 200 images.
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.