We are working on cloud computing and I have to choose a middleware like open nebula, but my doubt is how elastic open nebula is as compared to other middleware like nimbus or Eucalyptus.
Up to how many VMs can be installed or uninstalled in open nebula ??
Scalability of OpenNebula regarding a single instance depends on the underlaying infrastructure components chosen for storage, networking, etc, but usually we do not recommend over a 100 hypervisors running on a single instance. I recommend reading the reference architecture to get more in depth knowledge.
OpenNebula can also be configured in a federated topology to increase scalability.