Question: How many NMC have been burned for domain registration to date?
There are nearly 10,000,000 name_new
transactions on the blockchain. One single name_new tx burns 0.01 NMC. Therefore around 100,000 NMC have been burned.
In the early days, name registrations had to actually burn a lot more coins (IIRC up to 50), so the real number is probably higher - but I have no number by how much.
Technically speaking these 100kNMC haven't been burned, they are just inaccessible. If we ever run out of NMCs because of a future large-scale usage of Namecoin we could do a hard fork and purge all the expired, abandoned, unused or erroneously issued name_new transactions. This would release those inaccessible NMCs to be reused. A hard fork requires the approval of the miners/mining pools, though.
If we ever run out (which I doubt will happen and I'm pretty sure you don't think so either), we could also just hard fork to increase the block reward. That would have the same effect and might be a simpler solution; but anyway, that's not going to happen.