I recommend keeping only your active theme and an extra theme installed on your WordPress site at a time.
This keeps the files to a minimum but gives you the flexibility to quickly switch themes if you need to test something.
So, let’s get deleting!
Delete a WordPress Theme
► Login to WordPress, if you aren’t already.
► Click on Appearance > Themes.
This will take you to the Themes page. Your active theme will be the first listed and say the word, ‘Active’ to the left of the theme title.
► Hover over the theme you would like to delete and the click on ‘Theme Details’
This will take you to the Theme Details page.
► Click on the Delete link in the bottom right corner.
► Click OK on the ‘are you sure you want to delete’ prompt.
Congratulations! You just deleted a theme.