In this paper repetition-based cooperative diversity technique has been taken into consideration. For this technique there exists a trade off between the gains achieved by diversity reception and the decrease in bandwidth efficiency. Both effects have a hard dependence on the number of cooperating terminals. Here an analysis based on the average bit error rate (BER) of multilevel signaling MQAM modulation is presented to characterize the optimal achievable end-to-end throughput. In this way it is shown that threshold decisions can be taken to find out the optimal choice for the number of relays and modulation size. Thanks to that, efficient protocols can be designed using this information.