Upvote:2
The duration of stay is tallied day-by-day according to the rules detailed in How does the Schengen 90/180 rule work? If you don't need a visa, there is no restriction on the number of entries and no βadmitted until dateβ granted upon entry like in the US (i.e. you don't reset the clock by going elsewhere). So if you stay one week, leave and return, you still have 83 days.
If you were not a US citizen and did need a visa, the visa would specify the number of entries (1, 2 or multiple entries).
If you intend to use up the 90 days or have a complex itinerary, make extra sure you get entry and exit stamps every time to be able to prove you did not overstay. For a couple of weeks, it's less relevant of course.