Actually, I think it's 349 peasants in total, and 20 working in this particular building. To know how many are working in total, you'd need to select each building in turn and add them up. (Unless there's something I've missed that gives an overall summary?)
20/20 means the structure needs 20 workers to operate at full efficiency, and currently has all 20.
The others are serviles, workers (70), middle class (38), upper class (4) and aristocrats (9).