There's a hidden difficulty value generated every time you accept this mission. Higher tier difficulty values pay out more (but not so much more that it might be suspect)

The intent of this mission was for the player to do it immediately and not leave it in their logs for hours and hours and hours (which is why you can only accept one at a time) - when I realized players were doing this, I made it time out after 4 hours.

Here is exactly how the spawn mechanics work: at most, the hunters have a 50% chance to spawn. After they spawn, they are guaranteed to not spawn for at least 1 jump afterwards. The pirates will also not spawn in the player's home sector. The chance for hunters to spawn is halved after 1 hour (so if they had a 1/2 chance to spawn, it would now be a 1/4 chance) and then halved again after another hour (so a 1/2 chance would become 1/4 and then 1/8)

There's no halving of it again b/c, again, it times out after 4 hours.

The mission will ask you to kill anywhere between 5-22 pirates, with a the higher difficulties having a bias towards a higher # of targets. If the mission asks for 22 kills, it is possible that it is the higher difficulty one that will spawn bounty hunters.

The highest difficulty (50% chance to spawn bounty hunters on each jump - again, they are guaranteed not to spawn two jumps in a row) will never ask you to kill less than 10 pirates, so if you get a contract for 5-9 pirates, you're guaranteed to have at worst a 25% chance of running into hunters on each jump.

If being attacked by bounty hunters is an issue for you, just abandon the mission.