There's nothing set in stone in regards to the maximum amount of credits or cargo a pirate can demand from its target. However, there are occasions where a pirate who asks for too much will be in violation of this rule:
' Wrote:1. General Terms & Conditions
0.0 Be excellent to one another. Consider the people around you. Keep fair play in mind. It's a game and nothing is gained, or lost, save the experience. Credits can be re-earned. Ammunition replenished. If you become infuriated step away. If you make a mistake apologize. If you wish to remain in system after you die, or after someone has moved away, simply ask.
Fair play, a level head and consideration for others.
In general, demanding 100% of your cargo is okay by the rules. Pirates are allowed to do this. However, when it comes to credits, things get a bit more complicated. If a pirate demands a sum of credits several times greater than the total profit you'll make for whatever cargo you're carrying at the time, he's probably breaking rule 0.0. I won't go into exact numbers here; I'd probably come up with a lot of inaccurate "maximums" and "averages". And transports carrying ore are another deal entirely. I'm not sure how to calculate it all out properly.
If you feel that you've been unfairly treated by a pirate who asks too much of you, you can submit a report in this section for a rule 0.0 violation, where the Admins may or may not decide to take action. I'm sure they've got their own ideas of what's "fair" and what's "unfair" in a pirate encounter. If their views happen to match yours, the pirate will be punished somehow. If not... well, nothing happens. But it can't hurt to try, if you're unsure about it.