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.

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.