max-n-of reports an agentset that contains a specified number of agents with the highest value of a given reporter. For example, if we wanted to create a model where the largest 5 turtles are divided in half, we would write the code below:
ask max-n-of 5 turtles [size] [
    set size (size / 2)
    hatch 1
]
Things to keep in mind when using max-n-of: 
min-n-of primitive that reports an agentset that contains a specified number of agents with the lowest value of a given reporter.max-n-of will report the entire agentset. For example, if we have only 2 turtles in a model, max-n-of 5 turtles [ size ] would report all 2 turtles. In the model example below, we have some people who represent an absurdly simple economy. At each tick, a randomly picked turtle executes a trade by picking another turtle randomly and giving that turtle 5 of its money. This exchange is represented with a temporary link between the two turtles. We use max-n-of to change the richest three turtles' color to blue. We also use min-n-of to change the poorest 3 turtles' color to red.
Once you mastered the max-n-of primitive, don't stop there. Check out the resources below to improve your NetLogo skills. 
max-n-of primitive: