I’ve discussed this in the past (in some cases the ancient past), and when I left the hobby there were two theories regarding the amount of Shadowless product.
As background, printing companies always overprint (to account for printing losses), and almost always print to a contractual range. If WOTC wanted 1,000 cases of a product, then the contract might have specified a range of 950 - 1100 cases with WOTC willing to take whatver was produced. Carta Mundi (or any other printer) would have then calculated their likely internal losses and produced some higher figure so that the final amount of good product would be in the contractual range.
In addition, WOTC knew that there would be additional series of Pokemon cards to be printed. So I have no doubt that Carta Mundi printed huge amounts of sheets with the card backs, since that was a separate process, and would be the same for all cards.
The two theories for shadowless print size differ as follows.
Theory 1. The printer printed excess card fronts for the first edition print. In this scenario WOTC had a firm amount of 1st edition it wanted. The excess was held by the printer to be added to the base set unlimited run. When the new artwork for the base unlimited was issued, WOTC made a financial decision to allow the remaining “already printed” shadowless product to be issued with the base unlimited product. In this scenario the amount of shadowless was likely a lower amount than 1st edition, because the printer, even if building inventory (and even if the printer thought unlimited would be the shadowless art work) would probably not have doubled the size of the 1st edition release.
Theory 2. Everything from the initial print run was stamped 1st edition and released. But WOTC was overwhelmed by the immediate popularity of the game. The Base Unlimited artwork and corrections were not ready to be printed, but WOTC needed more product. WOTC had the printer make a second run with the existing (shadowless) artwork, but without the 1st edition stamping. Shadowless was sold until the Base Unlimited artwork was ready for prime time. In this scenario the amount of shadowless was a function of WOTC art approval logistics.
There was no concensus of which theory applied as of 2000-2001. We were hampered by the same problems as now. Shadowless wasn’t easily recognizable except to those who were sensitized to its existence. There was an early Ebay power seller (Gene was his first name, can’t remember the last) who specialized in shadowless in the early 2000’s. If anyone (non WOTC) had a clue about how much was made, it would have been him.