What is meant by the numbers on reagents above and below arrows?
The numbers indicate steps. The only problem is that the definition of a "step" can be slightly different. Sometimes, separate steps mean that you do something, wait some time, and then do the next thing. Other times, you need to work-up or destroy the excess reagents that are left and isolate the desired product before you can proceed with the next step.

In your example, you need to keep the mercury and the borohydride separate. If you used any excess in the first step, you might need to destroy it before you add the borohydride.