I should point out that the number 6 came from all sources in opposition to this deal. I don't think the administration ever said anything like "hey, it's only 6 ports", although I've read that here.
It is "interesting" that the number is 21, but I imagine the others aren't as comprehensive as the 6 -- there must be some reason why those 6 stood out for the opponents to harp on that number for so long.
This also once again points out that there are american companies running these ports -- they are simply subsidiaries of foreign companies. And after the deal, the same american companies will run the ports, reporting to the same people in the foreign company -- it's just that now THAT company's board will be reporting up to the board of a different company.
Because NY, Philadelphia, LA, MIami gets people's attention whereas Gulfport, Savannah, Brownsville would not.
And the unions are concerned about future contract negotiations with them.