The United States' Copa America quarterfinal in Houston, Texas against an opponent to be determined, has already sold 66,000 tickets. The president of the Houston Texans announced today that NRG Stadium is almost already at capacity (71,500) four days from the semfinal match-up.
The Chairman of the #Texans said that 66 thousand tickets for the semis of the #CopaAmerica have been sold so far. - via Bing Translate
Unlike the Seattle crowd that was predominantly American, you can probably bet on alot of the tickets that have been purchased are for the possibility to see Lionel Messi and Argentina in action. They face off against Venezuela in the opposite quarterfinal match to see who will earn the right to play the USMNT.
It's safe to assume that a lot of those tickets were bought in advance under the assumption that Argentina would make it that far. However, there still should be a fair amount of American supporters in the crowd.