Optimal amount of products per box and boxes to send

I sent 40 products to Amazon, mostly as a test. It took a very long time for them to be distributed to become prime, and it’s still not prime available everywhere, or still has longer lead times. How much product do I need to ship to get it available for two day shipping everywhere? Is 20 units in 20 boxes (400 total) enough or too little or too much? These products are small and cheap to store in Amazon warehouse, but what about larger products? I manufacture these items myself and am enrolled in brand protection, so I have a monopoly on the buy box.