The cloning attack is harmful to RFID systems. So, estimating the number of cloned tags is helpful to evaluate potential security risks for RFID systems. This paper studies the problem of estimating the number of cloned tags to present a cardinality estimation scheme CECT when there exist unknown tags and the capture effect. CECT scheme requires a RFID reader to first predict responses of the known tags by a virtual frame executed in terms of the ALOHA protocol. Then the reader collects responses from active tags over a channel with the capture effect. Simulation results show that under the given number of unknown tags and capture effect parameter, CECT can meet the required estimation accuracy and reliability. Under the same parameters, the accuracy is improved by more than 20%.