I am getting this from the chapter "White Servitude" from the Cambridge History of Slavery Vol III: AD 1420-AD 1804. The authors are William G. Clarence Smith and David Eltis. Some of the other stats that are eye-raising is the idea that of the million to a million and a half of the white slaves entering the Maghrib from 1530 - 1780, only 5 percent escaped or were ransomed. The tales and accounts of cruelty are directly quoted though from old newspapers from back then. The author(s) even mention that captive accounts are impossible to authenticate. Just how bad was it, or is this a reflection of contemporary European records which may have a bit of bias? Where would these numbers come from?
Heya, sorry for the delayed reply. This earlier answer of mine looks at the Barbary slave trade in this era, including conditions for the slaves.
The scope of the trade is a puzzle that scholars are still working to unravel. It does seem to have been quite sizeable (a million is a fair estimate; my linked answer talks about that, too), but the big question is--why don't we see more of an impact on Europe and the European economy from the such great loss of people? Or are we just looking in the wrong places?