G19 and G17 I have about ten factory mags for each as well as few assorted Magpul and ETS mags.
Guns that I'm unlikely to take to gun school or use for heavy shooting, I'm generally happy with four or five.
AR mags...I have no earthly idea how many of these things I've stacked up, but having lived through the Ban Era of '94-'04, I am not entirely rational about them.
My mom's parents got married in the middle of the Great Depression. They had a lovely late-19th/early-20th Century bungalow in Chicago with cut-glass doorknobs on half the doors. Every single one of the doorknob shafts was covered in rubber bands because they wouldn't throw any away. I'm like that with AR mags.