get_theme_mod outputs number when using WP_Customize_Cropped_Image_Control

I am not very familiar with Customizer part of code, but my immediate guess would that number is attachment ID for the uploaded image.

IDs are most typical and reliable way to store connection to particular media item within WP installation. It is trivial to get from ID to other representations, such as URLs to different image sizes. However it is quite challenging to go in reverse direction, such as determine ID (if any) for arbitraty image URL.

You can check if number matches up with ID in admin (typically exposed in URL) and use API functions on it (such as wp_get_attachment_image_src() and others).