The question as I understand it is how many mags should you have for a defensive rifle, not how many mags do you have in your load out or can use before you are killed. First off, if I was planning on getting killed I wouldn't be needing a rifle or mags, I'd already be outa there.
As I said, with mag and rifle bans a very real possibility, I figure what I currently have is all I will ever be able to get. My guns may never be fired in anger but who knows about my great grandchildren that inherit them. Therefore my mindset is "lifetime of the rifle supply" if I can afford it. Minimum just to keep the gun in the fight for a reasonable amount of time is 10 magazines, so if they are on the expensive side I would try for 20, a spare for each of the primaries. If they are cheap like aluminum AR mags are now, I'd go at least 30, 2 spares for each. AR mags are not known for durability.
Now my G3/HK91 clone has about 75, Tapco had a clearance sale on "slightly rusty" steel mags in 50 packs for a buck each back in the day. Most cleaned up with oil and some rubbing, the rest with a quick blast and paint job. Most of the others are their $2 Al ones. If they hadn't sold out I would have had another 50 steel, if you find a killer deal on magazines buy them if you can afford them even if you figure you have enough. Like anything else, magazine prices rarely go down, Al Hk mags are now $7 and the steels go for $9-20 each depending on condition. Used, you don't even want to know what new ones go for... If I had a few more of the 50 packs with a little labor I would be making at least a $450 profit per box, likely more.