At existing cloud computing environment, virtual machine allocation mechanisms have been actively applied to resources management for processing dynamic workload in large-scale data center. In this paper, we address the problem of how many virtual machines should be allocated to a server, so that a given percentile of the execution time of a job is bounded by a predefined value. We first consider the case of “dedicated CPU”, whereby we calculate the CPU allocated to a single VM. Subsequently, we extend the analysis to the case where the CPU is shared equally by different groups of VMs. In this case, we calculate how many VMs need to be allocated in each group.