Why do my slide numbers not appear in PPT 2019?

Here are some reasons why slide numbers do not appear in PPT.

A slide object overlaps the slide number placeholder on the slide

On the individual slide

To solve this select the overlapping picture and put it on the background. Under Picture tools select the Layout tab and click on the Send backward button, choose To the back. If the slide number is in an unobstrusive colour you may need to alter the formatting. Of course you can change the position/size of the picture also.

On the slide master

In the video below you can see an example of a picture which is on top of the slide number placeholder. Slide numbers are present but barely visible as the default colour of the numbers is light grey. So you can either adapt the size of the picture or you can alter the formatting of the slide number placeholder so that it becomes more visible.

The footer box is unchecked on one or more slide layouts in the master

If the Footer box in the Master layout group is unchecked on one of the layouts the slide number will not appear. Neither will the date or the footer itself.

To reactivate it go to View - Slide master - select the slide layout in question and check the footer box. Then go to Insert - Header and footer -  hit Apply to all.  Or leave the slide master by clicking on the Close master view button. select the slide(s) where the footer is missing and go to Insert - Header and footer -  check the slide number box and hit Apply. Not very intuitive but there you go.

The slide number placeholder has been deleted from one or several slides

Select the slide(s) where the slide number placeholder has been deleted. Go to Insert - Headers and footers - check the slide number box - click Apply.