How many banner sizes cover 95% of ad placements?

Five banner sizes cover 95% of ad placements, making them essential for broad reach in display advertising. These standard sizes are: 300x250 (Medium Rectangle), 336x280 (Large Rectangle), 728x90 (Leaderboard), 160x600 (Wide Skyscraper), and 320x50 (Mobile Leaderboard). Focusing on these five ensures your ads are compatible with most inventory across ad networks and publishers. While other sizes exist, they account for a much smaller percentage of available placements, making them less critical for initial campaign setup. Therefore, options like 3, 4, 6, or 2 are incorrect as they do not accurately represent the industry standard for maximizing ad visibility.