Only about a quarter of them ever sold, leaving about 7,200 of them on the digital shelves. That is, until they were stolen (or, as the museum put it, "transferred from the wallet without authorization"). If valued at their sale price the stolen NFTs would be worth €13.32 million (US$15.3 million), though it's hard to argue the thief could've ever sold them for that amount given the museum had failed to do so for several years.
The stolen NFTs were soon made even less appealing to prospective buyers when the museum un-linked the image files from the digital assets, and OpenSea blocked them from trading.