In this rule

Historical Rules · 26 September 2015

Rule 701.29a

Official source text from the 26 September 2015 Rules release

Some spells and abilities instruct players to vote for one choice from a list of options to determine some aspect of the effect of that spell or ability. To vote, each player, starting with a specified player and proceeding in turn order, chooses one of those choices.

Find this number in the current Rules