Mussels are sold by the pound. You're probably paying around $2/lb...assuming fresh PEI mussels are what you serve.
Over the course of a year, the mussels get bigger and smaller, so you're better off doing your dish by weight, rather than by count.
One Pound is generally a good portion size to start with. That math is simple, then you just need to cost out the other ingredients...butter, wine, garlic etc.
For most restaurants, the RTN on mussels is one of the best performers on the menu.
The perceived value for the guest is really good too! Even though it's a pound of mussels in front of them, the reality is that only about 35% is meat, the rest is shell.
Great food cost and RTN for you, but it won't fill the guests up....plenty room left for other things.