In this rule

Historical Rules · 16 June 2023

Rule 713.2a

Official source text from the 16 June 2023 Rules release

Some substitute cards list the names and mana costs of the cards they can represent. Exactly one of the fill-in circles must be marked to denote which card the substitute card represents. This style of substitute card was found in Magic products that released 2011–2018.

Find this number in the current Rules