Difference between single stage and multi stage centrifugal pump?

You should think of a multistage pump as a series of single stage pumps all mounted on one shaft with a single large case that holds all of them.

As with all centrifugal pumps, the capacity of any single stage varies with the amount of head (discharge pressure) that stage develops. More head, less flow. Less head, more flow. In any case the single stage has a limited amount of head development at any given flow rate.

To produce more head, multiple stages are operated in series so that each stage's head is added to that of the previous stage.

Multistage pumps can be either horizontal or vertical in configuration.