Could I really have been only getting 6% oxygen when in coma when I was a kid due to asthma attack? Could it have been 60%, not 6%?

60 percent. It seems you had a respiratory failure due to severe asthma exacerbation. You were very likely to have received 60 percent oxygen to maintain adequate blood o2 level.
Odd question. Room air is 20% oxygen and if you were being treated for asthma at a hospital you would never get less than that. Standard treatment would provide you humidified oxygen at concentrations needed to keep your blood level at or above 92-94%. Whether or not your body was able to take in & use the oxygen depends on the intensity of the attack & sometimes even 100% inhaled o2 doesn't get thru sick lungs.