It's said that the only way to be sure of having sufficient boat spares and equipment to hand is to tow an identical yacht astern. Not a very practical solution, but Murphy's Law dictates that you'll otherwise have a spare part for everything but for the thing that's broke.
Modern cruising boats are much more complex machines than they were a generation ago - it's impossible to have a spare part aboard for everything, irrespective of the size of your boat and the depth of your pocket.
But conversely, the increasing popularity of recreational sailing has led to the spread of chandlers and specialised service centres along most of the popular world cruising routes, and if your required bit isn't in stock it won't take long to get it brought in.
Reassuring to a degree, but Murphy is more inclined to amuse himself when you're far offshore rather than conveniently close to a well equipped chandlers.
So starting with the premise that you can't have a spare part for everything, how do you decide exactly what to take aboard?
That of course would be easy, if you knew what was going to break. The best you can do though is to form an opinion as to what parts of your boat are most vulnerable to loss or damage through accident or general wear and tear, and what the consequences of such failure would be.
Only then will you be able to objectively address the issue of what spares to have aboard. Could you live with the problem until you reach shoreside facilities, or would it need fixing at sea?
Here's the list that I came up with for Alacazam's Atlantic crossing...
Mechanical and electrical tools
The problem with tools is knowing where to stop...
Sails and rigging
Now I'm no medic, but this is what I've ended up with in my first aid kit:~