There could be a use for a role, even if you don't have anyone doing that role today - I may need Role A, not having anyone that can do Role A, but I can still allocate Role A to investments, showing the demand and how many resources I need to fill Role A.
Agree with Abisek and Michael regarding recommendations on business and project driven roles - project roles often don't equal job titles. Stick with project roles.
I've always been told "start with as few roles as possible - 10, 20 - you'll have a better chance of success." I can tell you from experience, that starting with a couple hundred roles leads nowhere good and that in a global company, getting down to a few dozen roles is very difficult.
Avoid requests to include skills and/or proficiency in roles (e.g. Project Manager-Software-Expert) - put skills and proficiency into Skills object, or avoid altogether. The proliferation of roles that this causes will kill capacity/demand management, portfolio management, ... and, it may even get one into trouble in EU where making skills/proficiency readily visible to many users violates their privacy laws.
Project based roles, less than 20, no skills/proficiencies in role names - keep it simple - can add complexity, later, but only if needed.