When you run a product import, magento will look for the image files declared in the spreadsheet and look for them in the media/import/ folder. It will then copy them across into the media/catalog/product/ subfolders. As customers browse your site, magento will then additionally create multiple resized files for each image to show them at the appropriate size as declared in your theme on the category, product, cart pages etc.
So, the number of images will increase each time you import. Depending on your theme, I would expect there to be around 4 or 5 copies of each product image, each one a different size. Once imported, you could delete the images in media/import/ as these are no longer need. You'll want to also be careful not to reimport images on each import as, depending on how Magento is configured, it could overwrite images in admin but not delete them from the server.
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