n-of is used when we want to randomly select a specific number of elements out of an agent set. It is similar to how one-of works. For example, if we wanted to create a model in which we wanted to have 90 yellow birds and 10 red birds, we would write the following code instead of using create-turtles twice:
create-turtles 100 [
    set shape "bird"
    set color yellow
    setxy random-xcor random-ycor
]
ask n-of 10 turtles [
    set color red
]
Things to keep in mind when using n-of:
ask n-of 10 turtles [move].In the model example below, we use n-of to pick 50 random patches to sprout 50 plants. We also create 5 cows to graze in this grassland. We also use n-of to make two of our cows violet.
Once you mastered the n-of primitive, don't stop there. Check out the resources below to improve your NetLogo skills. 
n-of primitive: