"Very many" isn't something I'd normally say either. It might be worth trying "a great many" which, to me at least, is more normal sounding English. However, I think Duolingo wants to make sure we're aware of the distinction between multas - many and plurimas - very many, so I would expect "many spears" to be marked wrong.
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."