You can only have as many cloned voices saved as your tier allows you to.
For instance, if you created 30 cloned voices and you downgrade your tier to Starter, which allows 10 cloned voices, then you will only have access to the other 20 voices if you upgrade again.
If you have a Professional Voice Clone (PVC) and downgrade below the Creator tier, which gives access to professional voice cloning, your PVC will stay in your library but you will not be able to use it until you upgrade to Creator or above.