Every few years, I bulk-up on neons to replace the part of the school that has perished. When I do this, I'll buy 5-8 at a time and put them in a QT. When I notice a symptom, I remove the fish immediately. Sometimes, one will die and I can't always get to it before it's eaten. So, I will end up making 2-3 purchases in succession. I am of the belief that NTD will show very quickly with neons stressed by the new environment and that the disease isn't easily transmitted without being eaten. I simply watch carefully for about three weeks and then move them to my display tank. So, I don't think there is any point in waiting to add more, unlike with something like ich. It's more important to watch carefully for a few weeks, and take quick action to avoid any risk.