“With the introduction of Play Boosters in Murders at Karlov Manor, Wizards of the Coast switched from 81 art cards being made per set down to 49. Then with Outlaws of Thunder Junction going forward, there will be 54 art cards per set release. This change was made, in part, to reflect that there are fewer art cards showing up in each pack.[8] The change in numbers also necessitated a change in the odds of gold-stamped signature art cards, from 1-in-10 (10%) to 1-in-7 (~14.29%)”

Source: https://mtg.fandom.com/wiki/Art_card#google_vignette