That's an incorrect generalisation and as you say later in your post, it depends on where in South America. There are regularly more than 4 seats available on the same flight (sometimes as many as 9) in Business to Bogota for example.
Fair enough. There's certainly never more than 2 on the South American routes I look at regularly. Ex-Madrid at least - I understand that there may sometimes be extra award space when starting outside of Spain.
All the more reason for OPs to be specific rather than vague.