If you include structures in which sulfur has an expanded octet and exclude structures with triple bonds, how many resonance structures can be drawn for sulfur trioxide, SO3

Seven

Explanation:

SO3 has a trigonal planar structure. The most apt bonding description is that of sulphur forming three sigma bonds giving rise to the plane triangle and then the presence of three delocalized pi bonds.

Plausible resonance structures for SO3 has been shown in the image attached. In all the structures, there is none in which sulphur is pictured as having triple bonds.

image credit: socratic