I believe to the order of millions of translations per day, but again, I don't remember where to view this, so might not be correct. But it's definitely orders of magnitude larger than 100s of users.
(And indeed, as a sibling comment points out, the feature is suggested to users in context, which of course massively helps with discoverability, so it's no surprise to me that it's used way more often than the extension.)
(And indeed, as a sibling comment points out, the feature is suggested to users in context, which of course massively helps with discoverability, so it's no surprise to me that it's used way more often than the extension.)