I am using mvapich2-0.9.8-15 version of mpiexec. I want to run a model named SINTEX-G in AMD 64 bit machine. I am using the command mpiexec -np 1 -max_np 6 oasis. where 6 is the maximum number of processor and 1 is the number of process to start. oasis is the executable. But iwhen I execute the script it shows as follows.

invalid "local" arg: -max_np

usage:
mpiexec [-h or -help or --help] # get this message
.
.
.
Any help would be much appreciated.

MPI implementations generally do not differentiate between "real" processors and nowadays cores. Therefore the command in your case should be "mpiexec -np 6 oasis" (assuming you have 6 cores).

Most MPI implementations will spawn n processes to get n cores working, some will likely spawn one process with n threads, some will do something in between or wholly different. But on the user side all will give you n crunching cores. How many processes a specific MPI implementation spawns to achieve this is generally unimportant.

Why is this an issue for you?