We used to say, if enough is good, then too many is better, Sensible Prepper Presents: “How Many Magazines Do You Need?”
It depends on how hard it is to get replacements. Magazines are expendable parts which wear out. When I buy a gun, I get enough magazines to last the life of the gun. Six for handguns and ten for rifles is a good rule of thumb.
If it is an uncommon gun with a proprietary magazine, better buy a few more. Buying a gun that uses magazine you already use? Bonus.
How much ammo? That is a question for another day.
*Photo courtesy of US Army