Why are there so many instances of postgres.exe listed in Task Manager ?

It's totally normal to find many instances of postgres.exe listed in Windows Task Manager.For example, in UNIX, you can "fork" a process. A typical web service system would show many active Apache processes and just as many SQL server processes. The basic idea behind this is that if a process dies, it won't take all other processes with it. Compare that to the usual multithreaded programming model in Windows where one thread can crash which would then cause the whole application to crash.