I suggested 8 because that appears to be the common value for other infantry units with the weapon, e.g. unit 94 in the British OOB and unit 64 in the Canadian OOB. Other nations with alternative anti-tank weapons of this period also tend to have 8 rounds and this isn't dependent on squad size.
I agree that 9 rounds for the PIAT is better for historical accuracy but it would generate more work across other OOBs. You guys work so hard on improving the game for us, so I'd happily let you take the easy option on this one
cheers,
Tim