How many layers does the average onion have?

One non-specific answer is that "It depends how big the leaves are on the plant. I believe there is one leaf for each ring and the bigger the leaf, the bigger the ring will be."

From:

http://wiki.answers.com/Q/How_many_layers_are_in_an_onion

If that is true, then the number of leaves becomes critical. It appears that number is variable, depending on growing conditions:

Photoperiod and Temperature Affect Sprouting of Onion bulbs (Allium cepa L.)

http://aob.oxfordjournals.org/cgi/content/abstract/66/2/179

And then there is this cool analogy:

http://www.youtube.com/watch?v=t9nJlXLF06k