Minimum of 6 to 7 in general for pistol.
I carry 1+3, total 4.
Bug out bag would have 10 odd mags for pistol, 6 for the AR (besides the one in the gun, and again 4 or 5 for the back up pistol. I use the same pistol type, so its easier with 14 mags of one type.
For a competition pistol, 7 to 8 'working' mags, meaning 10 or 11 in all, to cater for any damaged or malfunctioning piece.