How much tile images do you need

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.
Mazaika - (C) 2000-2017 by Boris A. Glazer.