The number of vCPUs you can assign to a single VM basically depends on the license you have (see http://www.vmware.com/products/datacenter-virtualization/vsphere/compare-editions.html) With 10 cores and assuming HyperThreading being active you could assign up to 20 vCPUs to a VM. However, it also depends on the hosts's workload (other VM's running) at which point overall performance will decrease rather than increase with more vCPUs.
André