What is the average number of netherrack per chunk? (In the nether)

In the Nether, the volume containing blocks is 16 * 16 * 128 = 32768. Depending on location, some chunks will be nearly full, others will have open space from lava lake to near the ceiling, so like 25% filled. You can ballpark this as half of this volume is Netherrack. So, roughly 16,000 blocks.