I uses what s needs at the time... I run Win10 on this rig. 16GB RAM. currently w/ all the crap I have running it is using only 3.5GB.
Even while gaming it never gets close to using all 16. I've even set my page filing as low as I can get away with and not have it freak out. 256 - 1024(MB) on C and 512-2048(MB) on D.
Never have issues.
Are talking about RAM or SSD space?