If you have a theme on your computer that you want to install, you can upload it to your Themes page:
The uploaded theme appears in the Unpublished themes section of your Themes page. You can install a maximum of 20 themes in your store. To publish the theme on your storefront immediately, click Publish theme. Your current published theme will become unpublished.
To see all the themes you have currently installed:
The featured theme shown at the top of your Themes page is your published theme. This means it's the theme that Shopify is currently using to display the pages of your website.
If you have other themes installed, they will be shown below as Unpublished themes.
You can have a total of 20 themes installed at any time. That's one published theme and up to 19 unpublished themes.