Assuming you would like to add multiple notes on your Mac computer, there are a few ways to do this. The first way is by using the Notes app that comes pre-installed on every Mac. To create a new note, simply open the app and click on the New Note button in the upper left corner. From here, you can type out your desired text and format it however you'd like. Once you're done, click File > Save to save the note.
If you want to add an existing file or document as a note, open the Notes app and drag-and-drop the file into the body of a new or existing note. This will automatically create a link to that file which you can then access by clicking on it.
Another way to add multiple notes is by using third-party apps such as Evernote or OneNote. These apps offer more features and flexibility than the default Notes app, so if you find yourself needing more than just basic text notes they may be worth exploring.