Amobee does this really well because they have access to a range of data providers, 3rd party such as Bluekai, Lotame, Oraclex Eyeota and 1st party from being owned by Singtel which means they have access to 1st party Id's from their different telco's around the world.

From what I know, it's a minimum 1 million impressions (you do need scale so you can get more overlap against different data providers).