Printers Pair:
Your artwork should 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.