What HW version is your router?
What FW version is currently loaded?
It's possible that you have hit the limit of this router however the options are set by a pool that is tied to other options that you maybe using. I wonder if you turned off options like WMM ENABLE, WDS Enable, Gamefule, might get you one or 2 more rules you can add. One the other hand you might have to just figure out what priority that your 10 devices needs and allow for only 8 devices, the other 2 will have to be manage in some other way. Not sure what the maximum rules you can set on this router.