Sorry Cavey but that isn't quite right. The 13 and 6 numbers represent the 19 buying options that are available from ALL sellers from the alternative buying choices on the right hand side. That is, they aren't stock level numbers, but the number of choices representing each seller as an alternative buying option.
See the list of all sellers (including one of Amazon's as a buying option) here: (6 New buyer options and 13 used Buyer options = 19)
http://www.amazon.com/gp/offer-listing/0764323008/ref=dp_olp_1?ie=UTF8&qid=1278080731&sr=8-2Amazon doesn't actually give out inventory numbers UNTIL their stock levels are close to running out. At that point an automated message such as or similar to this:
"Only 10 copies left. This item is being restocked"
will appear next to the In Stock message thus surreptitiously prompting the shopper to quickly purchase the item, no doubt.
So until that message shows up next to the green In Stock message I wouldn't be too worried about obtaining a copy.
Hope this clears it up.
EDIT: Here is an example of the message that shows up as only 5 in stock (but see how it says 22 new and 22 used underneath it)
http://www.amazon.com/BBook-Geek-Only-Humor-Youll/dp/0806530022
