rear cassette - which ratios

This may seem like a very stupid question by I have a 9 speed xenon cassette. are there set combintaions of rear cogs that go into that cassette or does it vary. What is the easiest way of working out which cogs I have at the back - do I have to count them?

 

You don't have to count them. There are numbers on them.
If a number is difficult to read you can always verify it by counting.

 

A problem with not enough fingers and toes?

 

Campagnolo says 12-23, 13-23 and 13-26 set combinations for their 9s cassette. I don't believe you can mix and match. It appears 9s is limited to 26 in the back. I doubt you could run a 29 from a 10s cassette, but I might be wrong (proper spacing?). IF you could, you may need a longer caged rear derailleur. Better to inquire in the forum.

I trust you're not counting by fingers and toes; not to get to 26, anyway.

-Wm.