Yes, eBooks are limited to a certain number of copies. This is dependent on the number of access licenses attached to each eBook, which is determined by both publishers and the library. The number of licenses is generally noted on the landing page prior to accessing the eBook content. If your access to an eBook is denied, wait a short while and try again - another user might finish with the eBook in the meantime.
The eBook Instructional Guide may be useful for more information. See the link below.