A previous title number is the title number of the title that the previous owner had.
For example if someone sells their car or vehicle to you and transfers the tittle to you then you get a new title in your name the old title would have the previous title number.
When you get the title transferred into your name you get a new title number and the old title would have the previous title number which is the old number.
Your title number should be on the title should be in the top left corner of the title and right next to the VIN number.
However, some states put the title number at the bottom of the bottom of the title.
The previous title number is invalid once you get a new title in your name as you'll get a new title number when you transfer the title to the vehicle to your name.