According to Netcraft's Web Server Survey, there are 876.8 million active websites. Netcraft collects information on sites' host countries, but only provides that information on a for-fee basis.

Absent hard data, then, we can make a reasonable estimate by looking at a large sample of the sites. According to a survey of the 1 million most-visited websites, the US hosts 43%. Even though that's not a random sample, it's not wholly unreasonable to assume that the US would be somewhere within that range for overall website hosting.

Combining the two data points, we arrive a crude estimate of 378,093,897 active US-hosted websites.