If a player is instructed to put a card into their graveyard without using the word “mill,” Bruvac’s ability doesn’t apply.

Many cards printed before the Core Set 2021 release have received errata to instruct a player to mill cards rather than to put the top cards of their library into their graveyard. Use the Oracle™ card reference at Gatherer.Wizards.com to determine whether a specific card has received such errata. As a rule of thumb, if an effect instructs a player to look at or reveal a card before it’s put into its owner’s graveyard, it’s unlikely to have received errata.

Because Bruvac is legendary, it’s unlikely that one player will control two. However, if that happens, each opponent mills four times as many cards as originally instructed. If they control three, their opponents mill eight times as many, and so on.