Ceph storage

It will be probably a stupid question but I haven't find the response.

In the wiki ( http://pve.proxmox.com/wiki/Storage:_Ceph ), you said we need 3 servers minimum.

- Why 3 servers ?

- Are the 3 servers need to be the same ?

- Can i have only 2 big ceph storage (mirroring) ?

Sorry for this question but i try to understand how it works ...