Yes I know this is strange im not all knowing and infallible .

My is this I had 2 slots for npc's to pop up in always have for some reason this morning
I now only have 1 .
what is the determining factor on number of slots open for npc's I have seen screen shots from the big guys that have 4 slots I had 2 and am now down to 1 .

Are you sure you're down to 1? If all of them were expired they only appear one at a time so you have to go back to the page to generate more. And one of the Glaviz missions gives you a battle that'll get you something that will increase your slots.

Im good for some reason it only showed 1 slot but when I refreshed it shows 2 and the 2nd slot is a Vortul, Crimson Annihilator for me to slaughter

Ya once one expires you need to refresh to see it again. Also Ferrus is right that one gives you two more spots but now there is a new mission out that gives you even two more after that. You won't see it till you hit rank 400 I think it is. Don't know for sure but I think thats it. So now I have six spots open.

Oh snap! Only 172 more levels to go!

It is part of the Zolazin mission chain available at level 25.

well I need to finish out the zolazin chain then

The one I was talking about is part of the Lazuli mission chain. And yes for those of you who don't know yet you can be that race too now once you do the missions for it.