Question

why computers have only 4 drives and why its name is c,d,e,f?

Answer 1

number of drives depends on how many physical drives your computer have (how many HDDs). also any physical HDD can be partitioned into several sections which will be seen as individual drives on the computer though it's only one drive(by the way, partition is often offered when you install your operating system)
So having c,d,e,f drives you can in fact have either 4 physical drives, one single drive partitioned or some mixed variant