I really dislike the "very many" translation of plurima if the sentence doesn't contain a negative. I suspect "very many" is a negative polarity item in English (see https://en.wikipedia.org/wiki/Polarity_item).
I suggested the very basic superlative translation "the most," which is not currently accepted. I'm also wondering: would it work to say "we have many many bones"? It's very conversational and wouldn't be taught in a grammar, but I don't see why not.
Many many would not be a good translation, as it's more like something a child would say.
The most wouldn't be accurate, as it would indicate that one could have no more of the specific object.
Very many means more than just many, but not the most. If we look at quantifiers comparatively, it would be many, then very many, then most.
Note that "very few" is used more often than "very many", but both are entirely acceptable as colloquial English.
It so happens that Duo will accept a word if it is within one character of an acceptable word it knows; often it will tell you you have a typo and move on, but sometimes it will silently ignore it. This latter behaviour is a bit unfortunate, but I cannot tell you when the typos are likely to be ignored.
In the present case, the word is not actually correct; in writing you must have the final a. In speech, as far as I have been able to gather from teachers such as Molendinarius or ScorpioMartianus on Youtube, when two vowels meet across a word boundary one of them tends to be elided (particularly the e in est) so you might hear a Roman say something like /ˈplʊ.ɾɪ.ˈmɔs.sa.ˈbeː.mʊs/ for plurima ossa habemus.