Working with more companies using ZOHO products. One or more ZOHO Users? User administration?

Working with more companies using ZOHO products. One or more ZOHO Users? User administration?

Hi.
I want to use ZOHO for 5 or more companies/projects.
What is the best way to use ZOHO regarding user administration.

Should I create just one User (one generell User for the whole ZOHO universe) and this user can be part of all the different projects / companies?
Or
Should I use one User for each project / company?

Or
Is it better to have one admin-user for changing the settings and one normal-user for working productivly?

Pleas tell me pros and cons.
Thank you!