Well, there really is about 400 syscall numbers allocated, and each has its own entrypoint in the kernel. That's an entirely objective criterion as opposed to arguing "well, sbrk and mmap (or recv and read) are kinda similar, maybe we should count them as just one syscall instead of two", because in the end we can might as well say "well, technically there is only one syscall, it just has an additional seventh argument that serves as the 'function denominator', like the ioctl's request parameter, you know".