If with "deck plate" you mean the same as the class rules with "inspection port", then yes, you can have any number of them almost anywhere, but that size may be too big.
As LaLi already pointed out, an 8" port is not legal. However, I doubt that anyone would protest you except for a Class Measurer or his/her representative.
On the other hand, it has been pointed out before on this Forum that an 'illegal" boat may be more difficult to sell later on.