The GTX 980 is a really powerful GPU. If you are a gamer that only uses one 1080p monitor, it will more than suffice for a long time to come.
The main reason why people buy two GTX 980's for SLI, is that they have multi-monitor setups. Or in my case, I have a 2560x1440 monitor at 120hz. I prefer to keep my framerate at 120hz, which takes twice the power of 60hz to push the ideal framerate.. Hence why I have two GTX 970's.
In short, if you have multiple monitors and wish to play the latest and greatest, two GPU's are faster than one.
If you have one monitor, you're easily fine with a single GPU, unless again you have a 4K monitor.