In English, you wouldn't say that. You would say "there are so many books" or "that's a lot of books"
An acceptable translation in my view.
For example, 'This is a lot of books for one person to read' would be understood, this and that being flexible.
The difference is proximity, not number. "Dieser" is this/these, whereas "das" is that/those, although the use of "dieser" is a bit looser than that of English "this". You can still use "das" for both.
Viele means many or a lot of, and goes for countable things as Bücher here.
Viel, means much, and goes for uncountable stuff.
I don't know about vielen as i use it with vielen Dank only. Surly could be used in other forms, but i don't know about those, yet.
"This is a lot of books" marked wrong for "That's a lot of books"...?? 'That is' IS same as 'That's'.
Why does Duo reject "are/is" and insist on translating "sind" as 'has" , when the notes say 'are", and "has" produces an unlikely English sentence?
I'm not sure I understand you clearly, but there is a subtle difference here between the English and the German as to how we conjugate the verb.
In German, the verb sein (conjugated here as sind) does not take an object. It must be nominative case on both sides, and we can swap these sides around without changing the meaning. A side-effect of this is that the verb always conjugates according to the number of the noun, rather than to whatever comes first:
Das sind viele Bücher = Viele Bücher sind das
Das as a pronoun is ambiguously singular or plural, so if you use it with a plural form of the verb you could translate it as "these" or "those" in English rather than "this" or "that".
Possible translations that I would expect to be correct:
"That is a lot of books"
"That's a lot of books" (usually Duo doesn't like contractions, but this is the default translation on the page I'm looking at)
"This is a lot of books"
"Those are a lot of books"
"These are a lot of books"
You'll see that in English, we conjugate the verb according to the pronoun, and don't have the flexibility of reordering the sentence without it sounding really strange.
I don't really understand what you mean about Duo insisting on sind meaning "has". That doesn't seem to have anything to do with this sentence, and I don't see it in the hints either.
YES, that all sounds good to me, too. But whenever I gave any of those answers DUO marked it as incorrect, and substituted "That HAS a lot of books" as the "correct answer".. It is not even a natural English sentence, unless maybe we are talking about a library. When I tried to skip, it just repeated the question, so in the end I just gave up and wrote "That has a lot of books" , so I could move on.
It might be because the "master" sentence in the database uses "that's", and then the database interpolates the contraction (out of context) as "that has", which is obviously incorrect. You could help fix it by reporting the error - the sentence discussion pages are usually just read by other users, who can't do anything about it...
Thanks all, for your helpful comments. Nice to know i have not actually gone completely mad. It was in a practice, not a lesson, so i can''t go back to it. Perhaps it was randomly generated, because no-one else had commented on it. But I did report it, a couple of times, so hopefully it will be fixed.