You can add up to 4 images per section, but you can use multiple 'Image gallery' sections on your homepage.
To add a new block, click 'Add image', and to delete any block, select the chosen block and click 'Delete' from within the block.
Each block has the option for a link, which can be to any page/product within your site, or any external link. To enter an external link, add the full address, i.e. 'http://www.shopify.com'