It is based on the Hosting Package you are using to create cPanels

Eg 1 : If you have a 15 GB Reseller  and you have 1GB Hosting Packages  : Then you can create 15 cPanels on your reseller account

Eg 2 : If you have a 15 GB Reseller  and you have 500 MB Hosting Packages  : Then you can create 30 cPanels on your reseller account

Eg 3 : If you have a 15 GB Reseller  and you have 250 MB Hosting Packages  : Then you can create 60 cPanels on your reseller account