Obviously, storage is very important. It is also important to know whether this storage space is your email?s, and websites or just the websites. If it is both that takes the storage up, then your storage will fill up quicker than if you just had to worry about websites. If the email is not counted, then it is likely that the host will set a limit on each email address you have.
As for website’s CMS? will take up about 100-250MB of space. The majority of your actual content will reside on a MySQL database which might also be included into your storage factors, but a MySQL will take less space than creating a unique page for every entry. Unless you are a web developer, you should really consider a CMS, and be mindful of the space, and what is included.
One thing to reduce the storage your CMS uses is to remove any themes or plugins you aren’t using. This could go a long way in shaving MB?s off of your storage demands. If your email counts against you, then consider deleting email from people you no longer write to. Email can definitely take up a lot of space, and you would want to make sure you can receive email, even if you do no more work on your website(s).