How many resonance structures can be drawn for #N_2O#?
The three resonance structures for dinitrogen monoxide are
All the three resonance structures account for the 18 valence electrons and show atoms that have full octets. Likewise, all three structures have the same number of covalent bonds - 2 sigma bonds and 2 pi bonds.
The difference between the structure on the upper left and the structure on the upper right will be made in favor of the latter by the placement of the negative charge. Since oxygen is more electronegative than nitrogen, the negative charge will be stabilized better by the oxygen atom.
The structure on the bottom will be a minor contributor because it has a greater separation of charge compared with the other two. Moreover, it shows a positive charge placed on the more electronegative oxygen atom and a very destabilizing (-2) formal charge on a nitrogen atom.
As a rule of thumb, formal charges bigger in magnitude than (-1) or (+1) will automatically imply that that resonance structure is a minor contributor.
Therefore, three resonance structures can be drawn for dinitrogen monoxide, with 1 major and 1 minor contributor.