What is the use of \n while scanning two integers like in scanf("%d \n %d")?

Read some good C reference website, and perhaps the C11 standard n1570 and probably Modern C.

The documentation of scanf(3) explains what is happening for \n in the format control string. It is handled like a space and matches a sequence of space characters (such as ' ', or '\t', or '\n') in the input stream.

If you explicitly want to parse lines, you would use some parser generator like GNU bison and/or use first fgets(3) or getline(3) and later sscanf(3).

Don't forget to handle error cases. See errno(3). Consider documenting using EBNF notation the valid inputs of your program.

Study for inspiration the source code of existing open source programs, including GNU bash or GNU make. Be aware than in 2020 UTF-8 should be used everywhere (then you might want to use libunistring whose source code you could study and improve, since it is free software).

If you use Linux, consider using gdb(1) or ltrace(1) to understand the behavior of your program. Of course, read the documentation of your C compiler (perhaps GCC) and debugger (perhaps GDB).