It'll depend on the specific CPU/Motherboard & GPU.
Some GPUs will have, say 4 outputs. For arguments sake, let's say 2x HDMI, 1x DisplayPort and 1x DVI-D.
You may only be able to use 3 of those at any one time (including adapters).
Coupled with your motherboard that may have, say 1x HDMI & 1xDVI. With the correct config, 5 monitors should be doable.
If you have mix of Digital (DP, HDMI, DVI-D) and Analog (DVI-I or VGA), you may find theres a limit lower than 5.