This great exageration makes no sense.
What would have been usefull is the amount of slaves working for me... full time (versus "involved" as described in their methodology).
What does it mean that 20 slaves work for you? Does it mean there are 20 slaves who at some point or another use products that you consume, or does it mean that there are an equivalent of 20 slaves working full-time for you (in terms of man-hours or something)? The former metric is rather meaningless since it's unclear what proportion of the labor of those 20 slaves benefits you. Is 1/1000th of their labor output? 50%? This makes a huge difference. The latter metric is meaningful, in that it would mean that you "employ" around 20 slaves. However the mathematics don't really work out in that case, since 20 slaves per person for hundreds of millions of Americans obviously doesn't make sense.
The way it's written as you point out seems to indicate that they're using the former metric, that is, the number of slaves involved in stuff you use, regardless of what proportion of their labor goes toward your lifestyle.