As far as gaming goes, one 1070 runs everything at 1920x1080 res which is all we need. Hell, my previous card (AMD HD 7970) did just fine at that res too on current-ish games. I really don't know why I would consider upgrading or buying more cards (I have 3 1070s now), except for mining cryptocurrency.
Maybe younger gamers (who don't remember playing games just fine at 640x480 in all its VGA 256 color glory) need 1440p or higher -- I certainly don't. God, I hate to be that guy, but I guess I am. :/ In fact, I would be happier at 1680x1050 with 16:10 aspect ratio for more height. But 16:10 lost the aspect ratio war long ago, and we just have to move on and make do with 16:9. A shame.