Dozens, possibly hundreds. There are at least 6 of them in Santa Monica , 3 more in West LA, about a dozen in Hollywood. And those are just the ones I know ab

Dozens, possibly hundreds. There are at least 6 of them in Santa Monica , 3 more in West LA, about a dozen in Hollywood. And those are just the ones I know about because they work with area startups. LA isn't even a major player in the advertising industry (either on the advertiser or ad serving sides)--SF/SV and NY/NJ are the major hubs.