Text boxes are always read by a screen reader after all content placed in the pre-existing ‘Content Placeholders’. More than one Text Box will really complicate predicting their reading order.
Also know that text placed in text boxes will not appear in the Outline view therefore complicating converting your PowerPoint to HTML or other formats.
PowerPoint contains built-in slide layouts that you can apply to any slide. When you use them with a new slide, these layouts automatically make sure that the reading order works for everyone.
It is recommended never adding text boxes. But, if you have decided to add text boxes to a slide, make sure the reading order of the text box(es) makes sense.