How much amoxicillin at 400mg can i give a five year old?

Not age based: The dose is not based on age, but on weight and condition being treated. Some conditions require a small dose of just 25mg/kg of body weight, while others require 90mg/kg of body weight. You should not be self dosing a 5 year old. That's what your pediatrician is for - their expertise determines what dose your child needs for their specific condition and body weight.