how to write a subscript or exponent on a number

Regardless of the version of Pages, the answer is on the Format menu : Font : Baseline sub-menu, where you will find subscript and superscript.

1. Enter the number or text that you want to be the subscript or superscript entity
2. Select just that character(s), and make your Baseline choice from above.
   1. The Baseline choice will remain in effect, until you select Use Default from that same sub-menu.
   2. Once you have selected Use Default, enter a space and continue typing in normal baseline mode.

Pages v6.1.1

Insert menu : Equation... (Enter LaTeX sub/superscript)

1. Subscript example

   N_2O
   

2. Superscript example

   x^{y+2z}
   
   

In TextEdit or Pages:

App menu > Format > Font > Baseline.