Most WWA steelbooks are shared print runs between multiple countries so that is not practical to number the steelbook itself
Sometimes the unsold stock from europe becomes available after release date and offered then to UK or another country for example a few months later
This why you sometimes see a UK sold out release become available again can be 3-6 months later or after 1 year
It's different when it's a same WWA steelbook release put into a slip as you are numbering the full slip or lenti slip etc not the steelbook itself
A WEA steelbook you could easily number as long as it won't be released in other countries at a later stage
Example UK
Dr Terror's House of Horror steelbook from Odeon Entertainment
It was only released in UK as a steelbook other countries was a FuturePak