WebPartManager in SharePoint 2010:
The WebPartManager sealed class is responsible for managing everything occurring on a WebPart page, such as the WebParts (controls), events, and misc. functionality that will occur in WebPartZones. For example, the WebPartManager is responsible for the functionality that is provided when you are working with moving a WebPart from WebPartZone to WebPartZone. It is known as the “the central class of the Web Part Control Set.”How many WebPartManager controls should be on a page?
In order to have WebParts on a page there has to be just one WebPartManager control to manage all the WebParts on the page.
