A good general guideline for carb intake would be to consume 5-8 grams of carbs per pound of body weight. Eight grams would be the high end for those who are performing a lot of exercise during the week and are completely depleting their stores, while five would be more moderate for those who are exercising but aren't performing as much volume. Do note that you will likely see some weight gain immediately after a carbohydrate loading period however this will most likely be water weight and will go away after returning to the normal diet protocol.
Arnold worked out six days a week back in the day because he could still make or maintain gains. Obviously when this was said “Train no more than 4 days per week.” it was targeted at “individuals who have a hard time gaining weight”. You don’t workout as much if you can’t gain weight. You need to burn less calories or eat more to gain weight while working out. Obviously you cannot workout 6 days a week and gain weight if you’re burning too many calories or not eating enough. Therefor, workout less days to gain weight because you’re not as awesome as Arnold.