we printed the dna microarray using agilent's ink jet process (agilent is a spin off of hp; imagine an inkjet with actg instead of cmyk). each spot on the array has many hundreds of thousands of molecules. after we cleave the dna off the array, we take a portion of that, amplify and the sequence it. the portion we took off we estimate ~100 molecules for every oligo we made (55,000). we the sequence to get >55 million reads (so 1000x coverage); but these are just averages, and the distribution varies. you can check out the supplement of the paper if you are interested.