Your local library imposes a limit on how many films you can watch via Cineast. The limit is calculated per calendar month or on a weekly basis (counted as seven days from your first loan). The number of loans you are allowed is indicated on the library's website or when you make a loan on Cineast.
It is your local library that decides how many films you can stream. They do this because they have a budget that needs to cover as many borrowers as possible.
Contact your local library if you have questions about the limitations.