Number of resonating structures (hypothetical)
Resonances structures are a model of the quantum mechanical orbital mixing.  To my mind they are a complete farce.  They are also quite useful

A model is a simplification of a problem, usually based on ignoring negligible factors.  So this question really distills down to what is negligible and what isn't? 

That depends on roughly two factors:


The conditions of reaction.  


Are the conditions strongly ionic or acidic?  Then charge separation may become relevant.


The rigor of the problem. 


Is this for a high school course? A Ph.D. thesis? A bar-room chat with your mates?

Unfortunately, like most of chemistry, there is not an absolute answer here.