It will depend on the age. I keep meal worms in a bowl in the tank so the gecko can find them easy and I know they are eaten if they are gone. I will also check the bowl 2 or 3 times a day when they are young and if it's empty I will put more worms in.
When the geckos are young I will put 5 worms in bowl, I do 5 because I can quickly count them and know they are eating. Once the gecko is older and eating well I will up it to 7 or 10 worms.
Once they are a month old they normally eat good so I don't worry about counting just put a pinch (10-15) worms in the bowl every day and check to make sure they are eating. I will also dump uneaten worms back into my worm bin and get fresh every day. That way I know the worms are well feed and ready to be eaten.
I will keep a small amount of vitamin powder and calcium in the bottom of the worm bowl for the worms to eat and crawl in. I might even put a small carrot in the bowl for the worms to eat while waiting to be eaten.