IPI stands for Interested Party Information.
An IPI number (referred to as a CAE number in the past) is an international identification number assigned to songwriters and publishers by their Performing Rights Organization to uniquely identify them as rights holders. If you are already affiliated with a PRO, you'll need to provide your IPI number when you sign up so that we can properly manage your publishing.
Your IPI number is available in the repertories of ASCAP, BMI, SESAC, and any other PRO's. They can be found by searching for your name in their respective repertory searches on their websites.
It should look similar to this listing