Biomax = 1 ceramic ring per gallon (3.78 L). As stated numorous times that's just a rough estimate but to be safe, at a very minimum one per. 1 biomax ring = at least 100 ft2 (9.3 m2) so that's a lot of area per little ring. I use about 3 per gallon, but I'm always like that
It is not possible to answer this question, there are too many variables.
First of, all ceramic rings have different sizes and different manufacturers
have different types of ceramic.
Ceramic filter media or other types of filter media have different surface size depending on type and brand.
The reason for using ceramic or other large surface types of filter media is
to give bacteria a place to break down waste to ammonia and then to nitrite before converting nitrite to nitrate (plant food).
So this means more filter media can handle more waste products from more fish.
As a general view on this, as long as they fit in the filter and don't block or disrupt the flow through it it's a good amount. I don't think there's really a set "x amount of rings per gallon" kind of deal, atleast not that I've ever heard of. I have these rings on the bottom section of my filter in my small tank and I've just stocked them to optimally fit the space they occupy without hindering the waterflow.
So a good sized filter for a tank will turn the water over around 5 times in an hour, or once every 12 minutes. So this should help you plan out an appropriate sized filter/pumping system.
Once you've done this, it's basically 'standard' to use 1/3rd of your filter capacity for biomass rings, this is the part which the bacteria in your filter will inhabit. The water should flow over/through this when passing through your filter - it's not enough for the rings to simply be on the bottom of the tank or such.
The other 2/3rds are used for sponges/filtering materials such as fibre mats / other additives.