When you decide to add users for all your Site Collections, you can't simply run through all the sites in your tenant. Why? Because what you "see" in the SPO admin center, is not reflecting what you really have. There a few Site Collections that are not visible. A few examples would be:
Author: Arjun Menon
Inspired by: Albert-Jan Schot
Author: Joseph Velliah
Author: Arjun Menon, Inspired by Patrick Lamber
Author: Matti Paukkonen
Author: Michaël Maillot
Author: Albert-Jan Schot
Author: Veronique Lengelle, Inspired by Salaudeen Rajack
Author: Patrick Lamber
Author: Nico De Cleyre, Inspired by Salaudeen Rajack
Inspired By: Salaudeen Rajack
Author: Waldek Mastykarz
Author: Waldek Mastykarz
Inspired by: Alan Eardley, Patrick Lamber
Inspired by: Alan Eardley, Patrick Lamber
Author: Patrick Lamber