Hi Mukund26,
Are you planning on creating a dedicated environment for Exchange or will you be leveraging existing resources. For example, you can't use Exchange without an Active Directory domain so if you're planning on creating a dedicated environment then you'll also need to stand up a domain controller virtual machine.
If this is just for a lab, you can likely create a single Exchange 2010 virtual machine with all roles. If you give it between 2 and 4 vCPUs and between 6GB and 8GB of RAM that is probably enough for a lab. I bet you could even assign fewer resources and have it work as well depending on what you're doing.
This is a good document on running multi-role Exchange servers. It doesn't specifically apply to your sizing question but has some good information nonetheless: Understanding Multiple Server Role Configurations in Capacity Planning: Exchange 2010 Help
Don't forget to leave some resources for your domain controller if you need to stand up a virtual machine for that too.
Hope this helps. It's hard to give general Exchange sizing guidance because so much is based on your requirements but in this case if it's just for a small isolated lab then what I listed should work for you. If you're planning on having actual users on the system then you may need more resources.
Matt
