On our cPanel servers the only limit to the amount of mail that can be stored is the storage limit of the hosting plan. So if the hosting plan has 5GB of disk space then a large portion of this can be used to store email. Of course if you have a Website you need to leave space for this too but plans can be easily upgraded if more space is required.
On the H-Sphere platform the storage capacity of each mailbox can be increased to the required level from within each users H-Sphere control panel. Please note that charges may apply when acquiring additional storage on the H-Sphere platform.