The answer might be "that's what it says on GTiROZ/Wikipedia". - It does say confirmed total of 21 though; I would have thought it's hard to corroborate (especially if Nismo VINs don't appear in FAST; which wouldn't surprise me as my RA isn't in there either), unless Nismo have unequivocally said they made 21 only.
As with the "Wintergreen", it seems like an odd number to me, but we're talking about a limited run of modified cars rather than a production run from the factory.
Out of interest; how many did Autech do?
That data in gtiroz/wikepedia, hehehe, is 10 or more years old, so probably needs to be updated. Shalom will get around to it one of these days.
My data base, which is only 70% completed, says that 47 Autech models were built, there may be some more to come, and I only have 3 affirmed Nismo versions. I know there are more as #5 exists in Canada.
Don't get me started on the 'wintergreen' tripe.