> 400 distinct syscalls is a very large number of syscalls 

Compared to what? It seems windows has 2000+ syscalls all combined (nt+win32). For just ntos there are 490.

https://github.com/hfiref0x/SyscallTables

https://github.com/j00ru/windows-syscalls

And does the number matter? Why can't it be 1000, 2000, 4000. In my opinion the interface is free to be defined as long as it is documented properly.