What is crtc and how many outputs can I enable using xrandr?

I'm using a Lenovo x220i thinkpad, running crunchbang linux (which uses the openbox window manager). The laptop has a DP and VGA plug on the side.

Hooking up the DP and VGA cables and running xrandr, I notice that the following outputs are available/active:

VGA1DP1LVDS1  <- the laptop screen

If I try to enable more that 2 (using any combination), then I get the error "cannot find crtc for output for [VGA1/DP1/LVDS1]"

What is crtc? How do I find out how many are available? Is it possible to add more in software or perhaps through a PCI card?

Thanks.