Most gamers should know this but RAM stands for Random Access Memory and I've included a video below which should answer all your questions. My installed RAM is 8.00GB and my usuable is 7.85GB. Sufficient enough for the video games I play.
My PC is equipped with 8GB DDR4 2133MHz RAM. It's enough for me and I don't feel I would need more any time soon. I haven't seen my RAM usage to pass 7-7.5GB when playing heavy video games with a browser opened. The 8GB of RAM fit perfect into my needs, the CPU and GPU are the more important things to consider when gaming. Also, you can shutdown some useless background processes or apps to keep RAM usage even lower.
I'm currently running around with 16 gigabytes of RAM in my system. I built the computer myself over Christmas and I was looking to upgrade from my rubbish old PC (1GB GFX and 8GB RAM) so that I could play some more high-spec games so I did treat myself to a nice bit of RAM haha. Least now I can play that resource hog PUBG