At that age I would likely feed him once a day but shortly I would start to cut it down to every other day depending on his size.
The vitamins come in a powdered form like the calcium and calcium with D3.
I dust lightly twice a month with a vitamin powder that contains a beta carotene prOformed source of vitamin A. I use one with a beta carotene source because prOformed sources won't build up in the system so you don't have to worry about overdosing it. This leaves you in charge of if/when you use a prEformed source. PrEformed sources can build up in the system and lead to health issues.
I dust twice a month with a phosphorous-free calcium/D3 powder to ensure that the chameleon gets some D3 without overdosing it and leaving it to produce the rest of the D3 from its exposure to the UVB light. D3 from supplements can build up in the system and lead to health issues but D3 produced from exposure to the UVB light won't as long as the chameleon can move in and out of the UVB at will.
I dust the insects twice a month just before feeding them to the chameleon with a phosphorous-free calcium powder to help make up for the usually poor ratio of calcium to phosphorous found in most feeder insects.
Proper basking temperature plays a part too because proper temperatures aid in digestion and thus play a part in nutrient absorption.
There is no appropriate number of insects to feed the chameleon. It's better to do as I suggested in my previous post.