You start with none. You find your first doubloon under the covered bridge (cross the river right to left), but you spend it on chicken feed for the boy's chickens.
Once you begin sailing among the islands, you find "salvage" floating in the water, especially above Dragon Cove and between Parrot Port an Golden Harbor. Sinking ships and sea creatures generates a lot, but is risky. Most doubloons will be made by trading -- buying the goods where they are cheap and selling where the price is higher.
(see related question)
There is no practical upper limit. Sinking ships or creatures with a large ship such as the Steaming Fury or Phoenix Warbird generates hundreds of thousands of doubloons per shipload. The likely "field limit" would probably be in the billions, more than anyone could possibly obtain even spending several weeks or months on the game.
You can get some by collecting salvage, especially around Dragon Cove. But once you have a ship with a cannon or two, you can sink sea monsters there and get lots of salvage from them. Of course, it is risky, because the monsters can gang up on you or pirates may also attack.
You can get doubloons by sinking other ships, monsters and by selling cargo.
keep trading
the cannoneer is 20,000 doubloons.
you have to get doubloons by trading grain silk medicine and spice (go around all of the islands on skullduggery)
Earn 500,000 doubloons then get a loan at Golden Harbor at the bank .Or you can as many as Grains as you can then sell it at Pirate Outpost until you get 1,000,000 doubloons.
it costs 20,000 doubloons
It takes approximately 25-100 "days." Everytime you land is counted as a "day" in Skullduggery Island.
Nothing, but you will still lose 10% of your doubloons. Sinking sea creatures is one way to create lots of salvage, but if you lose, you must be prepared to build up your doubloons again.
All you do there is to hire the Cargo Master, when you can afford him (salary 15,000 doubloons).
To get a large number of doubloons, sail around Dragon Cove (where you can sink sea monsters), or above Golden Harbor (where you will fight pirates). The accumulation of thousands of doubloons is a slow process, until you have large ships that can hold a lot of cargo.
"Cash" on Skullduggery Island is your doubloon count. You earn doubloons by salvage or by trading, and you can borrow some from the Golden Harbor bank. When you buy cargo, you spend your cash, and when you sell it, you make a profit. If your ship is sunk, you lose aprt of your doubloons and part of your cargo. You need doubloons to buy larger ships and hire skilled crewmen. (see related question)
When you have enough doubloons, from salvage or trade, you can buy a larger ship at the Shipbuilder on Dragon Cove.