Most (99%) of my games are downloaded and are single player. However I also play (massive) online multiplayer games like PlanetSide 2, The Division, and Star Citizen. Oh, and recently I started playing TriPeaks, although that isn't an online game ;)
I do have a few programs running while gaming; Steam or Origin or Uplay, MSI afterburner (for making snaps & vids), Logitech Gaming System (for having rgb keyboard profiles), Asus Audio Center, Asus AISuite (which controls the pump), and the usual driver & windows stuff.
It totals to around 1636MB in idle mode.
2070MB if I open Chrome, 2020MB if I open Edge both just 1 page. Chrome uses a lot more when opening multiple tabs but it does have a lot more extensions and overall is still easier to use.
No Man's Sky hasn't eaten more than 5400MB in 1440p. I even tried at 4K (Dynamic Super Resolution) and it didn't went above 5000MB, though the frames dropped to 30 max.
Indeed, any serious gaming rig needs 16Gb now. I found Forza Motorsport Apex to stutter on 8Gb and No Mans Sky is using 2.5Gb. I excpect when Forza Motorsport Horizons 3 hits in a month that'll be needing 16Gb too.