Google doesn't allow you to add more than a certain amount of accounts. It also doesn't allow you to log out of 1 account only. Very bad UX if you ask me. 

We can however get around this using browser profiles.

1. Click on the profile icon in the top right of your browser.

2. Add a new profile

3. (optional) import you existing bookmarks and passwords

4. Sign in to multiple accounts on the new profile.

5. profit?