Which chain should I buy to replace the one on my Specialized Rockhopper 2016?
New chains are a standard length more or less. You’ll need to remove some links to make it the correct size. Any 10 speed MTB chain is fine. KMC is a good brand.

You need a chain breaker tool to do this task. 

If you have used this chain for a long time there’s a likelihood that the chain rings and cassette are stretched too and need replacement at the same time. This isn’t literal stretch, it means the tooth pitch has been worn into the chains worn out pitch. Look at the teeth on the chainrings, do they look sharky? That’s a sign. 

You can check that with a chain wear gauge.

If the wear is a lot greater than 0.75% (most tools check 0.75 and 1.0) then you probably need the entire chain/chainring/cassette set changed. 

If your drivetrain is stretched and you only replace the chain you’ll get skipping when you pedal. 

You can avoid the full drivetrain replacement in the future by changing your chain once the stretch gets to 0.75. The cassette/chainrings will last 3-4 chains if you do that. 


Links aren’t recommendations, just illustrative.