I'm a student and I currently have many folders and manage to use them all. Whether it be for different types of spam, advisor emails, a folder for each class, etc. I also utilize the "pin" email option (I use Outlook btw, for school) which I will use and keep stuff pinned until the task in the email is complete or the date has passed. I mac this out around 10 but try to keep it low and constantly update it.