Absolutely! We even recommend it :)
With a single user, you can open multiple accounts and multiple websites per account. Here's how to do it:
To add multiple Websites under one account:
If you have a website with sub-domains (i.e. www.example.com, info.example.com and blog.example.com), you can add them under the same account.
From the main Accounts Dashboard, click "Add new Website" at the top left:
You will see that the Account is automatically selected.
Enter the sub-domain name and URL, and click "Create Project"
All websites you create under the same account will be grouped together.
To add multiple Accounts under one user:
If you have multiple accounts which are unrelated (for example, if you manage multiple websites for clients), you should create an account for each website (or set of websites as explained above).
To do so, click on "Add new Website" at the top left.
Then, click on "Create a new account":
Input the name of the new account, the new website name and URL, and click "Create Project".
* You can invite users to an account (this will give them access to all websites under the same account).
When you add multiple Accounts and multiple websites, your main dashboard will look like this: