Once the main quest is done,these seem to go on and on.repeats too.are these simply a way to get gold?are they limited in number.

Side note..never killed kern the vampire in the main dark quest.still open

There are infintate contracts dude, it just spawns generic NPCs so you'll never run out.

no there is no set amounts. and yes it is more or less a way to keep you making gold and stretch out the DB quest line.