I didn't think of max yet, but since Aternos has so many users I think there should be limits based on the size of mods. So maybe like 5 large mods(Orespawn) 10 medium sized mods, and 20 relatively small mods(Inventory tweaks). However you can select to have no large or no medium mods to increase the amount of smaller or medium mods you have. However I don't think anyone should have over five big mods.
Here is my table for exchange:
No large mods= +10 medium sized mods
Only small mods= +20 small mods(If needed)
I think this is a very efficient amount if the Aternos team tells me if this is possible or not that would be great, but at this current status this is just a concept that maybe can't even happen. Also if the Aternos team would want to change my numbers based on their hardware and resources.