We report the first systematic study of initiator density on surface-initiated polymerizations. We used mixed monolayers comprising of undecanethiol and omega-mercaptoundecyl bromoisobutyrate (1) to initiate the controlled radical polymerization of methyl methacrylate (MMA) and glycidyl methacrylate (GMA) from gold. We found that SAMs composed of 10% and 50% of initiator 1 grow PMMA brushes to approximately 1/10 and 1/2 the thickness of polymer brushes grafted from SAMs comprising 100% 1. We did not find a clear maximum initiator density, beyond which no further increase in polymer brush thickness is observed. We show that the initiator density at the start of the polymerization determines the ultimate "footprint" and hence density of polymer molecules in polymer brushes.