How do I limit the current supplied to a boost converter
Short answer - you don't, at least, not for the reasons you are suggesting.
You said it yourself...

I have a power supply that can supply up to 20 amps at 12 volts

That doesn't mean it will supply that much current. The PSU will only supply that much current if the load tries to draw it. If you don't want the load to draw that much current, then that is an entirely different problem.
What do you really mean by this:

I want to use the power from the boost converter to do anything I need
include being able to short the leads without the fuse on the boost
converter frying.

Do you really want to be able to short the leads and expect no ill consequences? Sure, an external circuit can be used to protect against this, but it's not a trivial task, and the solution has absolutely nothing to do with the converter input current or voltage, rather, it affects the output.
Have you considered just putting a cheap automotive blade fuse on the input or output lines? It will burn out if the converter or load starts pulling too much current and is easy to change. I don't think this is what you are looking for, but without more information, it's about the best solution I can give you.

Questions YOU need to answer

Since you didn't specify what you are trying to do with this converter, we can only assume you need to power something at a higher source voltage than 12V (your input voltage). You will get a much better answer if you give us some relevant information...

What is your desired output voltage?
What will the load consist of (What are you trying to power)?
Why do you need to limit the current?
What is your level of electronics knowledge?