When it states under each orc unit that one unit can be made big unz in the army, is it stating that 1 unit of orc boyz can be, 1 unit of savage orcs can be, 1 unit of boarboyz, or is it stating that only ONE unit out of ALL orc units can be big unz?

Only one unit in the entire army may be upgraded to big'uns.
It's on page 33 of the armybook.

Unless you run gorbad ironclaw as youre general, then you can take as many as you want