1 Database for multiple WordPress themes

If you’re going to be visiting clients to show them your templates I would personally recommend installing WordPress on your local computer by following the instructions on the below page.

https://managewp.com/how-to-create-a-local-wordpress-website-in-windows-with-xampp

Once you have installed WordPress locally you could make a backup of your live WordPress site and restore it on your local computer by following this guide.

https://codex.wordpress.org/WordPress_Backups

Doing it this way you’re not limited by your hosting providers 1 site only policy. You can also make modification without it affecting your live website in any way. You’ll also avoid the embarrasing moment of not knowing whether it’s a clients internet connection that’s slow or the server the website sits on.