In terms of mags... I consider a minimum of 3 per gun ...1 in gun, 2 on belt. However, in general, I want to have at least 6 for every gun...training, etc.
Ex: I keep 12 Wilson Combat 9mm mags, ETM, 10 rds -- on hand and I use them in 3 different guns. ( the same in Wilson .45 acp, ETM mags ). I usually pre-load my 12 mags before I go to range...and run my drills / carry them in a velcro mag storage pouch to keep range bag semi organized.
Speedloaders for revolvers... I have a minimum of two per model ( K frame, L frame, N frame 6 shot, N frame 8 shot ...per model ). I have several K, L and N frame guns...but I don't need a speed loader for every gun...