In this rule

Historical Rules · 25 September 2014

Rule 711.9b

Official source text from the 25 September 2014 Rules release

The face of each checklist card is divided into sections. Each section lists the name and mana cost of each double-faced card it could represent and includes a fill-in circle. When using a checklist card, exactly one of the fill-in circles must be marked to denote which double-faced card the checklist card represents.

Find this number in the current Rules