The only people that really know for sure would be Zavvi, Scanavo (the steelbook folks), or the studio.
Since we don't often get numbers from that group, here's actually what was printed, I think you can make an educated guess here or there.
1) Is the price for the steelbook cheaper than you would normally expect on a "new release catalog" steelbook? If so, they probably printed a lot of copies.
2) Is the price for the steelbook higher than you would normally expect on a "new release catalog" steelbook? If so, they probably printed less copies and charging more for each one as a result.
You can also play a guess here or there with the studio. Paramount (Star Trek) usually prints a large run, and if it's in other markets - like the same steelbook appears in Germany or something, then you know they printed a lot of them.