"The mirrors we most frequently encounter (not only in examples surviving in museums but also in representations of them in paintings) are from the Middle Kingdom, a period noted by the vast number of tombs which have been found. If we may judge by the inferior quality of the workmanship commonly seen in the surviving examples from that time, it appears that these objects were, in a sense, mass produced on an almost 'industrial' basis, in order to meet the immense popular demand for them."

Obviously, I've taken some liberties with the literal text in order to enhance what I believe was the author's intended main point: the high-quality of the craftsmanship of these ("liturgically" sacred objects, remember) which we saw in the examples from the earlier dynasties is no longer to be seen in those from the Middle Kingdom, which suggests that some techniques involving production shortcuts were introduced at this later date in order to meet the vastly increased demand.

The precise translation of the qualifying phrase "en quelque sorte" is not all that important; I have chosen to convey the essential idea using quotation marks around "industrial" --which further avoids the clear issue of the use of an anachronistic term, in the context of what must have been entirely hand-crafted production.

I prefer "Middle Kingdom" as a translation of "Moyen empire" because I believe that that is the term in most common English usage.

Btw, one might wonder exactly how we are to judge the quality of the mirrors seen in (merely) painted representations of them --but that is another issue we don't have to deal with here. We may assume that the author's assessment of the inferior quality of the mirrors of this period was based, primarily, on the actual examples which he had "before his eyes" (as the French like to say) and is in process of cataloging.

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Note added at 1 day20 hrs (2017-02-20 15:58:35 GMT)
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I assume that this section of the text is coming after the description/discussion of what the author takes to be the earliest examples of the mirrors he's cataloging.

Just to pique a knit:

If that is the case, then "Overall," might be inserted at the beginning of this sentence, in order to make it clear that, while there are some (relatively few) examples which may be dated (presumably on firm archeological, rather than stylistic or qualitative criteria) from the first few dynasties, the most common surviving examples --actual and painted-- date from the Middle Kingdom.

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Note added at 1 day23 hrs (2017-02-20 18:46:32 GMT)
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I see now that it is most likely the number of mirrors which is "prodigues," not the number of tombs.

So, "The mirrors we encounter most frequently ...are from the Middle Kingdom, found in vast numbers in the tombs of this period."

I'm assuming that a "prodigious" number is greater than a simply "large" one.