Congratulations that sounds very exciting!
It is very hard to say without knowing what your product is, what occasion it serves and who your target consumer is.
As a starter I would look at similar products in the supermarket and see what they do as a guide, you should also rely on insights from your store managers/buyers – go and ask them what they think is ideal.
You also need to strike a good balance between value for money and optimum price per kg too so please keep that in consideration. Also think about space in the chiller and freezer – you need to be large enough that your product stands out (pack format and design will help with this) but not so large that you take up lots of space as the chiller/freezer space is at a premium.
As far as units per inner go if it is a new product the smaller number per units the better – I would aim for your category minimum units per store per week that you need to hit without being delisted – probably something between 6 and 8. Again ask your buyer/store manager and look at what other successful brands in your category do.
Hopefully that helps.
Good luck with your new launch!