Is this a prospective build?
Post your parts list.
Decide what resolution you want to game on.
A single GTX980ti will game on a single 1440P monitor very well.
If , for some reason, you think you need more graphics capability, adding a second is an option.
Triple monitor gaming might well need a second card. But, there are not that many triple monitor enabled games.
For 4k gaming, a single GTX980ti may well be sufficient, it depends on the game.
For fast action games, a second would be very good.
Look carefully at any "cheap" 4k monitor.
Some will not support 60 hz with full color.
Having a second side monitor adds little load while gaming. it is a nice addition.