During the initial requirements and design phase of your application, the best practice is to create user categorizations known as personas. Personas consider the users' typical skills, needs, and objectives. From this information, you should also start to extrapolate their data requirements, such as the data they are responsible for creating (either directly or indirectly, by running processes), and what data they need to consume (reporting). Once you have done this, try to provide an estimate of the number of records that they will create and/or consume per month.
Share these personas and their data requirements with your executive sponsors, your market researchers, early adopters, and finally the whole development team, so that they can keep them in mind and test against them as the application is developed.