I don't know the answer, either, I can only offer anecdotal evidence. When I first got my Libra2 I made absolutely no effort to split my books into small batches. I sent all 500+ epubs (auto-transformed to kepubs) in one go. It took 30-40 minutes but there were no errors. It *may* be relevant that I know that all the transferred epubs are 100% error-free.
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