I've looked through several pages on bitnodes to find some more nodes that show NODE_WITNESS (12). I've noticed a pattern: the number corresponds to the services that a node offers. Nodes advertise the services that they offer by sending a bitmap named nServices. The number in the brackets matches the decimal representation of the service bits that a node has set.

Looking around a bit, I've identified the following five bits:

• NODE_NETWORK (1)
• NODE_GETUTXOS (2)
• NODE_BLOOM (4)
• NODE_WITNESS (8)
• NODE_XTHIN (16)

So, I'd say that with (12), you must be offering the services NODE_BLOOM, NODE_WITNESS (12), lemme guess, you run a pruning node? :) Or would it be "blocksonly"?

Edit: Here are the service bits defined in the source code: protocol.h:249