It's not in the FAQ because it is actually not often asked (!), and further it is very difficult if not impossible to estimate the casualties reliably. Keep in mind that many nations and parties engaged in the African slave trade, both legally and (eventually) clandestinely.

In both cases, shipping manifests may not exist, and when they do, they may be inaccurate or incomplete or falsified. Plus, bodies were simply thrown overboard as was considered convenient or necessary. And to avoid capture by authorities when trans-Atlantic slavery was banned.

TL;DR A terrible tragedy was made worse by actions of the perpetrators and enablers.

All that said, you want to look through this page which gives estimates and sources for those estimates. The estimated numbers seem to be somewhere between 9 million and 12 million, with much higher numbers appearing in several estimates.