Ook rekening mee houden dat niet enkel de plekken waar schepen kunnen aanleggen een kaainummer hebben. Sommige industrieterreinen hebben ook een "kaainummer".

The number should be in a ref=* tag. There's no question in that. The real question here is what other tags to use.

I also wonder how you can map the exact location of these numbers. Almost all the quays are unreachable.

"Possibly, the route could be added to the map data so it would show up on the walking map."

Only if the route is signed though :-) Don't know if that's the case here.