It would seem that the usual accepted standard is "iPod touches," but several may disagree.
If you do a simple Google search of your question, you will receive several results of sites where the same question was asked. The usual response was "iPod touches."
"iPod touch" is a registered trademark, so it can't be modified to become either "iPods touch" or "iPod touches". To make it plural, you have to call them "iPod touch devices". See Apple's trademark page:
"Remember that trademarks are adjectives, and cannot be made plural or possessive."