You need to clarify what Mac you're talking about (how many thunderbolt ports does it have?) and exactly what displays you are referring to (ancient-but-good Apple 30" Cinema display (DVI)? old 27" LED Cinema Display (DisplayPort)? current Apple Thunderbolt Display?)
If you've only got one Thunderbolt port (which is also a DisplayPort) then you can only plug in a single Displayport monitor, but if you have two Apple Thunderbolt displays you can plug the first display into the Mac and the second display into the first.
Confused yet?