Space required for snapshot in Hyper-V

Hi,

I have a query.

I have the following:-

1. BE 2012 on Win 2008 R2.

2. Hyper-V Host server on win 2008 R2.

3. 7-8 number of VMs running on each Hyper-V host server.

My query is how much free space is required for taking snapshot backup of hyper-v in the remote Hyper-V Host server.

Is there any thumb rule or its a  just the space of the VMs (all files ) is occupid.

Given a situation a backup of 2 TB for 3 VMS has to be taken, kindly clarify how much of free space is mandatorily required on the host server.

Thanks