On a monitor of single resolution of 1920 x1080p one card will always be enough.
If you think about future proofing, you can safely add another card, when you really struggle to play games of 2015 or 16.
If you SLI right now, there are a lot of games, who don't get better results with SLI so , i'm not sure it's a worthy investment at the moment on the single monitor of 1920 x 1080p.
Off course, if your monitor resolution is higher or you are looking for multi monitor setups, then the investment is really great !