I see on the site that licenses for 6+ servers will cost $39 each.

If I bought 6 or 7 now, then needed 1 or 2 more later, would I get the additional licenses for $39 each, or would I need to buy 6+ each time to get the discount?

You can purchase it anytime you want. There is a counter associated with your rvskin useranme, the one you enter while purchasing. You will automatically get the disocunt price, and don't need to purchase at once.