Answer: No, you don't! You can have one account and this account can act as a coach, manager and athlete. You can also have multiple clubs and act as a coach in one club, while being a manager and a coach in another. You can even be an athlete in a third club!
Your clubs will appear in your personal dashboard, which acts as a gateway for you to move between different roles.
PS! If you are not an athlete or you don't want to keep a training diary, you can switch it off in your "Settings" page.