How many resonance structures are there for the NO3- polyatomic ion?

There are three structures for the Nitrate ion. When drawing the lewis strcture you will have to find the bond order for each of the oxygen atoms attached to the Nitrogen.

Bond order = total number of NO pairs/ number of NO bonding locations.

Number of NO bonding pairs = 4 Number of NO bonding locations = 3

Bond order = 4/3 = 1.33.

This is shown as a double bond shown in the three different locations around the Nitrogen atom.

Here is a link to show how to draw them.