According to Official Guinness Records,
The most half marathons completed in a month by a female is 13, and was achieved by Stacey Kozel (USA) across nine states in the USA, from 4 to 27 June 2018. Stacey wanted to attempt this record in hope that her story would help someone out there. She also wanted to attempt this record to say thank you to her mother and all the people that have continued to believe in her.
For a complete list of 2018 records, please visit 2018 Guinness Records in United States.