I can feed my 2 month old veiled up to half inch crickets and he eats around 6-10, depending on how big they are, a day. Chances are, petsmart will have 3/8 -1/2 inch-ish crickets. They usually dont carry 1/4 inch ones.
I would be VERY CAREFUL feeding your cham as many as it would eat. Chams are known for being opportunistic feeders which means they make the most for the current situation. Think about it...in the wild they might come across a termite hill or a piece of roughting fruit with tons of flies. EAT!!!! Then for the next week there might be an increase in heat and insects are only coming out late at night. No food for days. Your cham will eat itself to death if you let it.
I would use your judgement based on the size of your cham as to how many food items it is offered per day. I would say for a younger cham it is harder to overfeed it. Five or so food items per day should be plenty. Remember, the more steady growth (feeding moderately) will be better for the development of the chameleon. If it is fed a lot, it will grow fast and many experienced keepers say these chams live shorter lives.
I feed Pluto who was an ambilobe 5 crickets early in the day and then offered 4 more in the evening until he was around 5 month old. Sometimes he would eat them and sometimes he would not so I would put them back in there bin. After 5 months I started feeding him 4 large crickets a day and a couple super worms