SHTF, give me a Finnish M39 Mosin and a bunch of stripper clips... If you get close enough that I need something with mags, the answer is simple... Saiga 12. For that, I believe that 4 mags is enough.
Seriously though, you should, for your one go to semi-auto SHTF rifle have 10 mags for it... Why? Because you never know... I'd rather have a few too many mags than too few.