How many resonance structures can be drawn for ozone?
Both structures account for the needed 18 valence electrons - 6 from 3 bonds and 12 as lone pairs placed on the oxygen atoms. The two structures are equivalent from the stability staindpoint, each having a positive and a negative formal charge placed on two of the oxygen atoms.
As a result, both structures will contribute equally to the overall hybrid structure of the molecule, which can be drawn like this
The hybrid structure, shown above on the right, will have two (-1/2) partial negative charges on two of the oxygen atoms and a positive (+1) charge on the third one.
Other resonance structures can be drawn for ozone; however, none of them will be major contributors to the hybrid structure. Here are two more possible resonance structures
These structures will be very minor contributors because, most importantly, both have an oxygen atom that lacks a full octet, and because there are fewer covalent bonds present compared with the other two structures, another factor that significantly decreases structure stability.
So, as a conclusion, ozone has two resonance structures that are major contributors to its hybrid structure, and at least two more that are very minor contributors.