How many surnames would it take to cover half the U.S. population?

Why the estimate? If you follow the links on the wikipedia page to the US census site, they have the data just sitting there. There is a big CSV file with the 150436 most common names. They even have a field with cumulative number of occurrences per 100,000.

Number of surnames it takes to get 50% of the US population: 2182

The 2182nd surname: JERNIGAN

When I was working in the U.S. I was surprised to see the great variety of surnames. I looked at the directory and there were about 200 engineers in the building. There was not a single repeated surname. It was interesting the great variety of origins. (e.g Polish, German, Spanish, Arab, Indian, etc, etc.) On the other hand a lot of people had the same names. (John, Mike, Dave) Here in Latin America is different, many people share the same family names, but we have more variety of first names.