The amount of storage space available to you is determined by your subscription. The Basic subscription plan allows up to 1GB of online storage for each year of subscription. The Unlimited and Premium subscriptions have no limit on storage space.
You can see how much storage space you are using by looking under the My Zenfolio tab in the Toolbox on your Home page.
If your account has a limit to how much storage is available to you, you will also see Storage Available listed. When you use up all the available storage you will not be able to upload any more photos. At that point you have two options:
For more information on subscription plans see the Pricing page.