In agreement with everyone else... yes bio balls can be used with fresh (or salt) water...
As IrnGynt described... what makes bioballs "nitrate factories" is when physical waste (i.e. poop) is allowed to collect in them. Since Bio Balls are not frequently cleaned, this physical waste continuously sits, rots and creates nitrates. Using proper mechanical filtration prior to bio media is esswential with any/all forms of bio media and it is important to change/chalen mechanical media freqently.
In essence, anything that colects physical waste and is not cleaned frequently is a "nitrate factory", and preventing physical waste from collecting in media will prevent it from being a "nitrate factory".
As for how many bio balls is necessary... bacteria will grow on any/all surfaces in a system. The more fish arwe in a system, the more ammonia is produced thus the more bacteria is needed. So there is no X volume of bio balls per Y gallons of water that can logically be applied.
In my personal experience I do not use any bio media and my tanks have ample surface area to sustain bacteria colonies to accomodate my stockign levels. In the hobby at large, we kept tanks just fine before manufacturers developed bio media for us to spend money on.
In the rare circumstances where we have so many fish in a system that the standard items in the system do not supply enough surface area, then bio balls or other bio media will give the bacteria the extra space it needs. But in my experience this situation is uncommon.