According to Arthur D. Jacobs, author of the autobiographic book "The Prison Called Hohenasperg: An American boy betrayed by his Government during World War II", by the end of the war, 11000 persons of German ancestry were interned, both immigrants and visitors. Also, under the pressure of US Government, Latin American countries arrested more than 4000 German Latin Americans, from which most were shipped to US for internment. At least 2000 from both groups were exchanged for Americans that were held in Germany.
If you ask why didn't they intern all Americans with German ancestry, such try would be simply impossible, as there were too many of them. That's why Germans were interned as individuals, not as a whole, like Japanese.
In 1940 there were more than million of persons born in Germany, next 5 millions whose both parents were born in Germany and 6 millions with at least one parent born in Germany. I've got no source for the overall amount of American citizens with German ethnicity during the WWII, but that would have to be even bigger.
It's well described with citations in Sheridan Report, written in 1980 by the US Government analyst, in order to clarify the reasons for huge disparity in numbers between German, Italian and Japanese citizens of USA, who were interned during the war. Here are the key points of it:
In December 1941 and January 1942 three Presidential Proclamations were signed, "to regulate the conduct and movement of enemy aliens":
In February of 1942, the mentioned Executive Order 9066 was signed. Month later certain groups of enemy aliens were excluded from that order. The list of them contained such conditions as e.g. old age, poor health or close family members of US Army soldiers. It was clearly stated that all those exclusions could apply only to aliens of German or Italian origins, not to Japanese, except for the poor health condition (deaf, blind or hospital residents).
Around the same time Henry L. Stimson, Secretary of War, stated that Italians are "potentially less dangerous, as a whole, than those of other enemy nationalities" and that "the size of Italian population and the number of troops and facilities which would have to be employed to deal with them, their inclusion in the general plan would greatly overtax our strength".
Similar conclusions were brought up by House Select Committee investigating the evacuation of enemy aliens. The linked material is the full text of Comitee hearings with many paragraphs speaking of Germans and Italians situation. From the reasons mentioned above it stated that "Indeed, this committee is prepared to say that any such proposal is out of the question if we intend to win this war".
From this reasons it was decided by General Hugh A. Drum, commanding general of the Eastern Defense Command, that "Mass evacuation is not contemplated. Instead thereof, such evacuations as may be considered necessary will be by selective processes applicable to enemy aliens, or to other persons deemed dangerous to remain at large within the area or within its zones".
General John L. DeWitt, commanding general of the Western Defense Command, disagreed with that, finding the mass evacuation a "military necessity". He demanded "definite instructions to the contrary that would exempt him from all responsibility for the consequences".
On May 15, 1942, he was informed that there was to be no "collective evacuation of German and Italian aliens from the West coast or from anywhere else in the United States", but that the War Department will authorize individual exclusion orders "against both aliens and citizens under the authority of Executive Order 9066".
As for Italians, in November 1942 it was announced that they were no longer considered "aliens of enemy nationality".
Jacobs provides the following map of interment camps for German-Americans.
The list of documents connected with the topic is listed and linked at German American Internee Coalition website.
The full timeline of interments and personal stories of interned people can be found in the links. As there are no sources provided, it's up to you to consider it valuable.
You may also want to read Wikipedia article dedicated to that matter.