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.
  
 
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