At the beginning of each year (or define your own year in gaming...doesn't HAVE to begin on January 1) you take ALL of your
backlog...and zero it out. So if on December 31, 2019, you had 72 games on your
backlog, as of January 1, 2020...you have ZERO games on your
backlog.
From that point on, every game you purchase counts as -1. Every game you complete counts as +1. That includes ANY game that you complete for the first time in 2020, even if it's a game you bought previously and forgot about, or never got around to, or whatever.
(You can also do your accounting in reverse...count each game purchased as +1 and every game finished as -1. It works out to the same, it's just a different way of looking at it. As long as you're consistent on how you keep track, it doesn't matter which way you go.)
There are also a lot of resources out there to help you keep your
backlog organized. Lots of people around here like
Backloggery. I like
Grouvee, as I prefer the interface and overall style. There are others, too.
Howlongtobeat is also a really nice site to help you sort through your
backlog, as you can see a good estimate of how much time you'll have to invest in each game to finish it. So if you want to beat some more games but don't know what to tackle next, it can be helpful to see that
Xenoblade Chronicles 2 takes the average person 65 hours to beat just the main story, but 112 to do the main story plus some extras (this is probably the standard play style). While
Wandersong takes 9-11 hours whether you're casual about it, or a completionist.
Sometimes knocking a few short games off your
backlog to get those precious pluses can be a morale booster

(Though it can be a morale
crusher to do all your short games at once and then only have looooong RPGs left to work on the rest of the year, so
howlongtobeat can be really good for this...figuring out just which of your games would be best to tackle next.)