If not use simulator or devstack, but use real production cluster, very necessary need will cost how many hosts(or nodes)?
CloudStack: 2 (management-servers and DBs) + 2 (Hypervisors) + 1 Storage(If you do not have a Storage Device, maybe you need a server for NFS or iSCSI) Total: 5 servers for a minimal environment with load-balance and HA. OpenStack: It depends on the component you have chosen. Every component can be installed in the right one server. But you need one more server for load-balance and HA. Total: 2 servers for a minimal environment with load-balance and HA.
When planning a cloud platform, the total resource = ManagementServer*2 + Hypervisor*N + Storage(Server Or Storage Device) Hypervisor number is the total cpus and memorys of how much vms you planned to run. Storage is how much volumes you want to allocate for all vms.