There's a ridiculous exploit in Bloodborne that lets you make more of any stackable item. Here's how to do it.
Mind, if you're looking to get the True Bloodborne Experience™, I would suggest steering clear of doing this exploit — it can make things way easier than they're supposed to be.
With that out of the way, the process for duplicating items in Bloodborne is surprisingly simple.
First, you're going to need an alternate character. Doesn't matter what you spec this character or what they look like — just make a new character. Progress this character enough so that they gain access to the first lantern in Central Yharnam, and hop into the Hunter's Dream.
Next, you'll want to go back to your main character. Here, you'll want to empty your storage chest — either take the items with you, or sell them. Whatever works best for you.
Now go back to your alternate character. Buy a pebble — yes, a pebble; I swear this isn't a joke — and store it in your chest. Make sure that the pebble is the only thing stored in the chest of your alternate character.
Swap back to your main character. You want to decide what to dupe. The exploit only works on items that you can have more than one of — so things like weapons can't get duped. But consumables can be duped. This means that you can duplicate items that give you Blood Echoes, the game's main currency/experience points.
Once you've decided what you want to dupe. Put ONE of that item into your storage chest with your main character. Now go to the store at the Hunter's Dream, and buy pebbles. Lots of pebbles. The amount depends on how much you want to dupe the item. The way the exploit works is, it takes any of the extra pebbles you can't carry, and it stores them into your storage...as what ever item is is you want to dupe/have stored one of in your chest. So say you're carrying 20 pebbles already — the maximum, meaning that any additional pebbles bought automatically goes to storage. If you buy 10 extra pebbles, you will gain 10 extra duped items in your chest.
Here's a video that shows this part of the process:
As you can see, I start with one Coldblood Dew, and I end up with 10 extra. I only had to spend 100 Echoes for it, and theoretically, I could have acquired way more Dews. But, I was only trying this out for demonstrative purposes — I didn't want to give myself too many Echoes!
Welp, there you have it. This is the sort of thing that will likely get patched out in the future, since previous Souls games have had the same happen with other exploits. But for now, it's in the game, and you can take advantage of it, if you'd like. Try to not get carried away, the beauty of Bloodborne partially lies in the difficulty!