There's no limit to the number of contributors (users who upload files into the system) or clients (users who will be downloading from the system) who can register on the system.
Depending on your plan, however, the number of admins and contributor users who can use the platform might be limited. Check out our plans here and feel free to contact us if you'd like to extend the default user limits.
When you sign up for a LightRocket plan, the system is yours to work with and it's ready to grow to meet your demands. Beyond the storage quota included in your plan (which is extendable), we don't set limits - because we want you to get the maximum value from our platform.