Ice_Mage: I keep a local list of my library and wishlisted items. If you've already amassed a sizeable collection, I believe Playnite can export your library to a list.

On the site, just hover over your name at the top to see your library total. In GOG Galaxy, see Owned Games in the sidebar (or if you own games on other platforms, I guess first filter for Platform:GOG.com then save the bookmark), but note this doesn't include non-games like The Witcher Goodies Collection or Cyberpunk 2077 Goodies Collection. Then compare the two totals every now and again.