Depends on the stain

Wood stain is different than paint in that some stains will actually perform worse with multiple coats. As a rule of thumb...

One coat

Olympic clears, toners, and semi-transparent stains only require one thin coat applied.

Two coats

Olympic solid colour wood stains require two coats for optimum durability.

Why?

Applying a stain moisturizes wood in a way that allows for the wood to soak in freshly applied layers. So, if you apply a second (unnecessary) coat of deck stain to a wood staining project that is already adequately covered, you risk creating a tacky surface that is prone to early peeling because the second coat is not penetrating the wood surface. It is simply layering on top of the first coat of stain.