I've seen the number one spelled "youn" and "en". What's the difference?

Yeah, you have en, youn, and yon
The only one you can most comfortably use before a noun (modifying a noun) is YON.
example:
Yo gen yon pitit → They have one child.
Ou gen sèlman yon chans → You only have one chance.

You use "EN" when counting
example:
en - de- twa
one-two-three

"YOUN" does not usually modify a noun.
for example, we don't say YOUN liv, we say YON liv (one book)
We use YOUN instead of YON at the end of sentences, and as an object pronoun.
example:
Mwen rete youn.
I have one left.
You cannot say Mwen rete YON or Mwen rete EN.

Mwen va chwazi youn ladan yo.
I will choose one of them.
so, you cannot use EN or YON here.

So, you'll say:
Ban m youn.
Give me one.
or 
Ban m yon pòm.
Give me an/one apple.

YOUN is also used in the expressions:
Youn apre lòt → one after the another
Youn dèyè lòt → one after the other
youn sou lòt → one on top of the other
youn pa youn → one by one
youn ou lòt → one or the other
youn lòt → one another
youn di lòt → one tell the other
youn te ede lòt → one helped the other, they helped each other
etc...

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