This NASA image of Messier 106 provides a good example of the difficulties involved in determining the number of arms in a spiral galaxy.
Messier 106 has two arms which are invisible at optical wavelengths but very visible at radio and x-ray frequencies.
I gave the Messier 106 example in a blog post last year but the image I linked to today tells the story a bit more clearly.
Thanks to Universe Today for drawing my attention to this image.