More often than not, you may have several virtual machines based on same software stacks running on the same host. Although they are very much the same, they take as much space as multitude of what one virtual machine takes.
Since vSphere 4.0, things are different. You can significantly reduce the storage usage by a new feature called linked virtual machines. The idea is simple: sharing a common virtual disk among the similar virtual machines. The shared virtual disk serves as a base. On top of that, each virtual machine has its own delta disk. When a guest operating system writes to disk, the data persists to the delta disk. When it reads from disk, the delta disk is checked first before trying the base disk.
As a result, you only need to save one copy of the base disk no matter how many virtual machines you have (up to 8 virtual machines in a linked virtual machine group). One limitation is that you cannot use it with HA cluster.
How to create linked virtual machines? You have two approaches: clone a virtual machine either from a snapshot, or from its current running state.