What does "11-25" mean in the cassette description? I need to replace the chain and cassette on my bike soon but I don't know what teeth to count.

The Shimano Ultegra 6800 11 Speed Road Cassette - has 11 gears on the back so I know to go for the 11- bit but I don't know the second part - 

e.g 11-23t 11-25t 11-28t 11-32t 

I think its 25 but what am I meant to count ?

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The first is then number of teeth on the smallest and the second is the number of teeth on the largest cog.  

If you like the current gearing then stay with that.  Or you can adjust.  If you stay with the same then match the number of chain links.

The count is also typically also stamped on the cogs.  On the bike and dirty it is typically easier to just count them.