Rule of thumb is "feed once per day the same volume as the head of the turtle has".
For the question What food that should be it's important to know what type of map turtle it is and if it's male or female. Please edit your question!
Female map turtles have to eat more plants than males. And there are sorts of map turtles who eat mostly algae or moluscos or fish and so on. The feeding depends on the seasons too. In winter (cold water, slow turtle) it is different than in summer (fast, hunting turtle).
In general fresh food is the best ever. You need no extra vitamins if you give greens and/or fresh fish/snails/bloodworms (baby mosquitos).
In general it's also a good idea to make fasting days in a weekly rhythm.