First off: the book mode and facing sheet settings apply only to how the document is presented on-screen, as does anything else under the view menu. Anything under the tools menu however will affect the output to the final PDF.
As for the number of folded pages in a signature: the number of pages is always double that of the number of pages per sheet side, since there are two sides to a sheet. So, a 2-up imposition, when folded has four pages; an 8-up imposition, which gets folded 3 times, ends up with 16 pages; etc. Looks like what you need is the 8-up imposition.
You can simply open the PDF and then change to the imposition. You can dynamically change between impositions (and no imposition) at any time.
The preferences for new documents are meant to be used if you tend to create documents with specific settings that are different from the default settings. Good candidates for customisation here for instance, are the margins settings for even/odd pages, to make sure the output always falls within your printer’s margins. You would only change the default imposition mode if you usually want a particular imposition and do not need to edit the individual pages before switching to the imposition.