Extended Clip isn't stackable, you have to go to the Drum for a larger increase.
In any case, what it "requires" has to be considered as what has to be done to have it and have it be balanced. Which means we'd be looking at more than a simple rules change.
That said, if you're looking for a house rule, my suggestion: Extended Clips have the same availability as the weapon, with the caveat that the restrictedness of the weapon is increased one step. Price them at ~50-100¥. Concealability is at +2 if they're loaded into the weapon, and it will not fit into a Concealable or Quickdraw holster. Given their size, something akin to the Combat Load Vest or the MOSLTOV vest from War is required to carried more than just the one in the weapon.