They need to make up numbers to have a reasonable format... otherwise there are 10 teams. They could invite Suriname or Guyana, but this will not help TV money etc in the same way as inviting Mexico or Japan.
I think 2 groups of 5 would be a reasonable format, with 4 games for each team:
But instead of the top 2 qualifying for the semi-finals (which could lead to 2 teams qualifying well before the end of the group stages), how about the top team in each group qualifying for the semi-finals, and the 2nd and 3rd teams in each group qualifying for crossover quarter-finals to determine the other 2 semi-final places.
For example:GROUP A
Brazil would qualify for the semi-final
Argentina would qualify for the semi-final
Then there would be 2 quarter-finals:
Chile v Colombia (winner to play Argentina)
Uruguay v Paraguay (winner to play Brazil)
Alternatively, the winners of the 2 crossover matches could automatically play the team that they did NOT play during the group stage to avoid repetition of matches.