If it's 12 pages and you make a booklet it will on be only 6 pages as it's in spreads.
But for a 12 page booklet you need to print pages 1,12,2,11
Those four pages will wrap around your 8 pages
Also for the Print Booklet feature - you need to tick Print Blank pages in two places
1. In the File>print booklet and under Setup - tick Print Blank Spreads
2. File>Print Booklet and go to the Print Settings at the bototm, in Print Settings under General tick Print Blank Pages
Hope that helps
Now for some home truths
Imposing is not your job, it's the printers job - if your printer requires this then they are not a good printers as this is standard for printing companies.
You really should use File>Export and choose PDF (print) and select your settings from there and then use the Print Booklet feature from Acrobat Print booklets | Acrobat, Reader
If this is for home printing, then it's fine - I'm not going to argue with your methods... But consider the option of Exporting to PDF and printing booklet from Acrobat as above.
Printing to PDF loses a lot of information required for professional printers - as it's passed through postscript which means it loses lots of smartness.
You lose transarencies, anything done in the fx panel, including blend modes.
Colour management is lost
Tagged PDF for accessiblity is lost
And loads of other reasons why it's a bad idea to print to PDF.