Sony president and CEO Kaz Hirai has announced that roughly half of PS4 owners are PlayStation Plus subscribers.
Sony announced earlier this year that there are now 7 million PS4 owners. This suggests that around 3.5 million of those owners have PlayStation Plus too.
PlayStation Plus is required to play multiplayer modes of non free-to-play PS4 games. Not to mention the service also gives you access to other benefits such as free games and discounts.