In the original pilot "The Cage" number one was a woman played by Majel Barrett. They dropped her and moved the actress to the role of Nurse Chapel as they didn't think the public was ready for a woman first officer.
Nice to find a fellow Trekkie. I recently caught up on Star Trek Discovery. In season 2, Rebecca Romain reprised the role as Captain Pike's number one. Picard also named his dog number one.
The year before he signed, Andrew Luck's dad was the starter, Giff Nielsen was the backup, and Peyton Manning's dad was third string.