Slide Number is added to a slide automatically using the option 'Insert -> Slide Number'. If the slide
number option is checked, the slide number is added to the footer of a slide. Add Slide Numbers in Microsoft Powerpoint 2002
In the above example,the slide number 1 is added by checking the 'slide number' option.
To use the the slide number in only one slide, 'Apply' can be used.
To have the slide number in all the slides, 'Apply all' can be used.
This helps to avoid slide numbers in specifed slides as you wish.