Water quantity in simulation box

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Hello users
I am new to md simulations
my question is
I have a protein with molecular wt 150 which has experimentally studied in water, the final concentration was 5 mg/ml
so I need to mimic exact in md simulations in which protein with 30-monomer single chain, so how much should be the water around the protein in 9nm box size?
can you please resolve the issue