If im correct about my guess, they did include the number in the second volume: https://www.amazon.com/OCD-Exposure-Coloring-Book/dp/B087HDKN78
Fear not, there is a volume 2
This is awesome! There’s a volume 2 too: An OCD Exposure Coloring Book: Volume 2 https://www.amazon.com/dp/B087HDKN78/ref=cm_sw_r_cp_api_i_2eepFb3486V2N