It was the three major houses: Raith, Malvora, and Skavis.  It's implied there are also some minor houses that haven't been named:

As to the number of vamps there:

We then get a number on how many are seated, and that is about a hundred:

There's some ambiguity there.  It may have been only a hundred total after all, or there may have been some vamps standing instead of sitting.  In any case, most of them were only the "leading members of the three major Houses", which would mean there were a bunch of Whites who weren't in attendance.

I think that maybe you’re looking in the wrong place. I think thank when Thomas initially tells Harry “what he can’t tell him about”, he describes it as a gathering of a hundred of the most powerful vampires of the White Court. And I’m assuming that would be politically powerful. So the number of attendees was probably the proverbial tip of the iceberg.

There is an interesting idea lurking around this conversation.  If it is the first fatal feeding that turns a white, does the infant who kills his mother during gestation come out a full fledged vampire?

And this is covered to some degree in PG at the Fool Moon Garage.

I suspect that you’re reading it wrong. If a fetus killed it’s mother by feeding, then it wouldn’t be born. I think that what is implied is that the feeding weakens the mother to the point that she doesn’t have the strength to handle the physical stress of childbirth. Making it a 50-50 proposition.

I'm further going to suggest White Court vampire who was pregnant could bolster her own reserves simply by feeding more often to strengthen herself. That seems pretty obvious to me.

I’m pretty sure that Thomas was referring to a 50-50 chance for mortals—since he was, after all, directly referring to Justine’s situation.

I agree, but if he were, she shouldn't have gotten pregnant. 

Exactly. If ya’ll don’t mind, I’m going to suggest a different tack. First I’ll point out that the naagloshii referred to Lara and her sisters as phages attached to a rotting meat sack (iirc). But he/it was speaking to them. That implies that their consciousness is the phage, and the body just happens to be along for the ride. Thomas referring to it as a “demon” is just a figure of speech, distancing himself from his inherent nature.

More importantly, I’m going to refer to the fact that Thomas states not only that they “took precautions”, but that his kind are “all but infertile”. In essence, a very low sperm count for males. I’m not going to speculate what the actual mechanism is. Nor am I going to question that Thomas and Justine took effective precautions. I'll take that at face value.

Instead, I’m going to go a different direction.

I’ll point out that throughout the series, there have been random events that have happened without explanation. The car that rear ended Harry, and all he concluded was that it was a crime of opportunity. Karrin’s car getting blown up, which he tried to attribute to the various villains of White Night, but it doesn’t quite fit. And on the flip side, times like when Marcone shows up out of nowhere with a Valkyrie—at the exact moment that Harry is destined to die.

Or, most specifically, when in Proven Guilty, after Harry has used Little Chicago for the first time to find Molly, Bob discovers that Little Chicago had been “fixed” with so that Harry’s head wouldn’t explode—then it was returned to it’s original state, with the damaged coupling. Implying that someone not only knew about Harry’s lab, about Little Chicago, but had the power, knowledge and access through Harry’s wards to fix (and unfix) it.

So I’m going to suggest here that someone, some outside force, meddled…to make sure that Justine got pregnant.

Not going to speculate as to how or why, positive or negative. But it seems to me that particular leap is a whole lot more likely for Jim’s style than… “Oops. It was an accident”.

I'm afraid that I'm going to have to make a couple of posts to add a few relevant thoughts...

Heres my take, and I think they played with the ordering in the trailer a bit to make people think

(click to show/hide)

That one of Marcone's people died.

Please pardon me doing some trimming. But I'm not sure of one of Marcone's people actually died or not, we just know that he appears to be the offended party. And that simply could be because he is the official host.

What we do know is that "blood has been spilled", which we saw as a Very Bad Thing when Mab has decided that everyone should play nice--and it's her Accords.

Matter of fact, the "blood spilled" may only be Thomas's--because he clearly got his backside kicked, whatever it was that he tried. Though it does appear (and that may be the key word, because...Jim) that Thomas was attempting to kill someone. If that was Marcone, I think we can guess who handed out the beating.

Scene 4:
Not necessarily people who got the invite, just the ones the trailer had the budget to display.

For the rest, I'll simply refer to Chapter One.
 
Scene 6:
People are drawing some odd conclusions from this scene. It's not as if Harry hasn't gotten dressed for a formal occasion off in a room with someone else before.

Scene 9:
Just to play Devil's Advocate, how do you know that isn't Goodman Grey? Or, remember the Skin Game trailer, where Harry was fooling around with Murphy? We all know how that one turned out.

Scene 11:
What I see is Molly reaching out to touch an invisible barrier (though perhaps commentary said more elsewhere). White Council Security--specifically Carlos, IIRC, has been known to put those up around meeting places before. If Molly is unable to get through, it would be one way for Jim to exclude one of Harry's (now) most powerful allies at a time when he needs all the help he can get. Much the way that Mouse is usually now sidelined with Maggie duties.
Scene 12:
The gravelly voice makes me think Mr. Ferro. (sp?)

Scene 13:
After he snaked his way down a narrow space (ventilation shaft?) to get to Thomas--and his tuxedo pants got caught on something on the way?

Scene 17:
Remember that in the Skin Game trailer, the production was limited by budget to "similar" settings, some of which were pretty distant from the actual setting. I think the point here was just to show Harry squirming down somewhere singing the Spiderman theme.

Spoiler Battlegrounds Description:

I found myself wondering. If the naagloshii were the small fry imprisoned on Demonreach, where a Titan would fit in the pecking order of the powers that are entombed in crystal there.