Your artwork for booklets must be supplied as printers pairs, the most simple way to work these out is to make a mock up of your intended booklet using blank paper and simply write the numbers on the pages,
A four page booklet is one peice of paper folded in half, the outside pages when viewed as a flat sheet would have page 4 on the left and page 1 on the right as below, turn the paper over and you will see pages 2&3. Page 2 will be on the left and page 3 on the right, these are your centre pages. Please refer to the example below.
You'll see I have worked out an 8 page and a 12 page booklet for you, you can work out bigger booklets eg: 16 page, 20 page etc by simply doing as I suggest and make one up with blank paper and write the page numbers on, lay the sheets flat to see your printers pairs.
4 Page Booklet laid out as Printers Pairs.
8 Page Booklet laid out as Printers Pairs.
12 Page Booklet laid out as Printers Pairs.
3mm Safe Rule.
Remember the 3mm safe rule.
No text or detail should be placed closer than 3 mm to trimmed edges or folds, this ensures no loss of detail etc in the folding, trimming process.