Rosa,
You can turn it off on alternate pages manually.
1. Switch to Slide Sorter View
2. Select alternate slides.
3. Click View | Head and Footer.
4. On the Slide tab; tick Slide Number.
5. Click on Apply.
That should do it.
If you are using PPT2002 or later you can set it up to use two different
masters on alternate slide. One master which displays the slide number and
the other which doesn't.
Regards
Shyam