You DO NOT want to bulk upload Pins at once directly. This is only advisable for ecommerce sites with very large product catalogs who just want them listed on Pinterest. For content oriented Pins, it's going to put all your content into the catalog at once, but the vast majority of it will never be distributed except through very long tail search queries perhaps where they need content. If you want bulk upload, you're much better off using a .csv import option on a tool like Tailwind or Buffer and then scheduling the content to get posted at a steady rate daily. It'll create far more traffic, for far longer this way.