There are two different types of email notifications you'll get for revision requests and/or comments on your projects.
First Comment/Request
The first time someone adds a revision request or comment on a project, you immediately receive an email with that request. This is so you know that you have revisions or comments coming in.
Daily Digest
The daily digest is a list of all revision requests/comments for that project within the last 24 hours. You will get one for each project that had comments or requests that day. These emails are designed to show you progress on each project for the day.