That depends on how you design the array and for what purpose. For example the size of the needles used to make the array will determine how many cores can fit.  If you use a 0.6mm needle you can fit more cores than if you use a 2.0mm needle. You should have at least 3 cores from each sample, and several different types of tissue as positive and negative controls arranged in a way that will make it easy to orientate yourself when viewing under a microscope. Chapter 6 of this book describes some important considerations for building tissue microarrays and may help you in your design (http://link.springer.com/protocol/10.1385%2F1-59259-759-9%3A061)