I'm not sure on that, I don't really know all the details personally either.
There's no sure way really to check either as:
Say with Fullscreen if they do have 20 ad sales staff (they could have much more) I'd say 7-8 would be the actual sales agents.
If each of those 8 agents were to sell $75,000 each in campaigns in a month = $300,000 at an average rate of a $20 CPM ($12 after YouTube's cut) that's 30,000,000 monetized views only.
Now Fullscreen gets 6,000,000,000+ views a month so 30 million views is around 0.5%
So even if you were to watch 200 of their videos a month you would only have 1 chance of actually seeing an ad.
Those numbers are speculation but likely not far off for the top networks that are even serving ads.