I usually just let a bunch (10-15? it's kind of hard to count them) go in the cage. My chams will eat what they want and then clean them up the next morning.
You can get some food from mantisplace.com or make your own from the Kammers recipe:
FLY FEEDING INSTRUCTIONS
Flies are a favorite food source for many chameleon species, especially babies!
Hatching the Fly Pupae:
The flies you ordered will arrive in the pupae stage. Store them in a warm spot (80-85F+) until they hatch. After you notice them hatching, let the culture stay out for about 24 hrs. 350+ flies will eventually emerge. Once the culture is full, place the flies in the fridge to cool down and they will go into a semi-hibernated state. You can store the flies for approx. 3-5 days in the fridge without loosing many. Any longer than that, they tend to die out. Try to put them in the warmest spot of the fridge (i.e. the fridge door at the bottom).
Gut loading:
Since the flies emerge with empty stomachs, you will want to "gut-load" the flies to make them nutritious for your chameleons. You can mix a soft-boiled egg (mashed to a smooth consistency) with a few teaspoons of plain yogurt, baby rice cereal and some honey. Once cooled, take the culture out of the fridge and knock the flies down by tapping the container to one side. Carefully open the lid and smear a generous teaspoon full of fly food to the side of the container. When the flies warm back up to room temperature (within minutes), they will immediately begin feeding on the yogurt mixture. Two to three hours later, they should be ready to feed out. From this point forward, the flies can stay out. You can gut load any remaining flies the next day, by following the same procedure listed above.
Feed out Tips:
Here is a helpful tip if you don't have a front "sliding door" on your chameleon cage . . . after the flies have gut loaded, put them back in the fridge for about 5 minutes, just prior to feeding out. The flies will be crawling from being cooled down and you can easily release them in the cage without them escaping everywhere. Prior to lifting up the cup lid, hold the container as close to the cage top as possible. Lift the lid SLIGHTLY on one end (the cup lid edge furthest from you) and lean the cup against the side of the cage. The flies will crawl upward on the screen side. Close the culture lid while it’s still in the cage, then quickly pull your hand out and shut the cage door.
Happy Feeding!