why does "Under the toga are very many gems" work, but not "Under the toga are many gems"?
"Very many" and "many" are both uncountable, and mean "a lot."
The course creators still needed a way to distinguish between the adjectives "multus, multa, multum" and "plurimus, plurima, plurimum."
For whatever reasons, they decided on "many" for "multus, multa, multum" and "very many" for "plurimus, plurima, plurimum."
"Very many" seems like more than "many," even if no one can ever tell you exactly when "many" becomes "very many."
FWIW: A good writing coach will tell you to never use "very."
I will try to explain. This is comparative usage of an adjective, so it is much like the -er, -est endings in English. The degrees go as follows: multus, -a, -um (many); plus, pluris (more); and plurimus, -a, -um (most). The above are direct translations; however, when we say "this is the best!" in English, for example, we don't always mean it literally.
Therein lies the caveat.
The superlative degree in Latin accounts for hyperbole in that it can mean "Most" or "Very."
"Under the toga are very many gems" is bad English. "Under the toga there are lots of gems", would have been a good solution. "Plurimus" is in this course always translated with "very many", which is itself an unusual thing to say in English. The primary meaning of the word "plurimus" is listed in my dictionary as "most". I get the feeling that the use of it here is questionable.