To add to what Stefan has said (a) there is a hard limit of 32 linked text
boxes in a Word document, (b) just adding a page with a text box at the end
won't help if you're creating a booklet; you'd have to insert four pages,
and you'd have to insert them at the correct places. For example, suppose
you have an eight-page booklet and want to add a page. The current
page/sheet layout is as follows:

8 and 1
2 and 7

6 and 3
4 and 5

For a 12-page booklet, you'd need to have this layout:

12 and 1
2 and 11

10 and 3
4 and 9

8 and 5
6 and 7

You can see that you would have to do a lot of wholesale rearranging just to
add a page. It is much easier to use Word's built-in "book fold" feature if
it is available to you; otherwise, as Stefan says, the method described at
http://word.mvps.org/FAQs/Formatting/BookletPrinting.htm is more efficient.