Traditional works rely on the use of complete Channel State Information (CSI) at the base station to take space-time scheduling decisions in the uplink of multi-user MIMO wireless systems. However, this might be a complex and costly solution. On the other hand, the simplest scheduler is the Round Robin (RR) scheduler that schedules terminals one after the other without using CSI. However, when multiple antennas are used, many terminals could be simultaneously scheduled because of the additional spatial dimension. The optimal number of terminals to be scheduled by a space-time scheduler with no CSI in a multi-user SIMO system is investigated in this paper. Furthermore, we also study how spatial diversity and multiplexing gains are translated into throughput gains. In particular, when a ZF receiver is used, we show that the benefits of using multiple antennas increase with the minimum SNIR requirements and that, when the number of antennas is much higher than the number of terminals, the throughput gains due to multiplexing increase linearly with the number of terminals and exponentially with the minimum SNIR requirements.