Flesh hounds of Khorne are typically used to rapidly hunt down opposing troops. They're not vehicle killers and they're not monstrous creature slayers if we're being honest. But they could do both of these roles (e.g. rhino killing) if other daemon troops have already been wiped away and one is desperate. But there should be superior daemons choices for either of these roles. I regard their primary role as to engage opposing units rapidly, and tie them up. Hopefully they might score a few wounds as well. But how many should be used?
Against Space Marines.
Let's put some flesh hounds up against space marines and see how they perform. An experienced daemons player should be able to take advantage of the flesh hounds and ensure that they take few (zero?) shots on their way in to combat and that they get the all-important charge in.
If they get the charge in, then they will strike first due to the furious charge special rule. They will get three attacks each, hitting on 4+, wounding on 3+ and killing when the marines fail their 3+ armour save. Hence each flesh hound will cause a statistical average of 0.33 wounds on the charge. Therefore three flesh hounds will be required to take down a space marine.
At this point, we can see that we're going to need a lot of flesh hounds to take care of the space marines, even with Karanak coming along to help. At 15 points each, I think that the points spent on flesh hounds will only tie up opponents rather than deliver a killing blow. I therefore think the points would be better spent elsewhere, unless you're playing a mono-Khorne army. And even in a mono-Khorne army, I think I'd only have a unit of flesh hounds in it for their speed to tie opposing units up. And their fluffiness. They're simply not that good in my opinion otherwise! (Sorry to sound so negative!)