Skip the "or more" part, then, but that doesn't make it less true: it wasn't possible to make a 6-argument system call with syscall on alpha 8 years ago. I don't know whether that's been fixed or not.

The syscall(5) manpage documents the Alpha syscall abi as passing arguments in a0,a1,a2,a3,a4,a5 which would suggest so. (MIPS o32 is the only listed one which is a bit oddball: you can only pass 4 args in registers and then use stack for 5 and 6.)