Realistically, a medium urban planter box will comfortably fit two pepper plants, and in the best-case scenario, you might squeeze in three compact dwarf varieties. The key is not just width and length, but soil volume, airflow, sunlight, and the mature size of the pepper variety you choose.
If your “medium” planter box is roughly 24–36 inches long, 12–18 inches wide, and at least 10–12 inches deep, it likely holds about 20–35 gallons of potting mix. That sounds like a lot, but pepper roots still need room, and crowded plants compete for water, nutrients, and light. As a general guide, each standard or semi-dwarf pepper plant should have about 5–10 gallons of soil and 18–24 inches of growing space. Compact or dwarf peppers can often manage in 12–15 inches of spacing and 5–7 gallons each.
So for a typical medium urban planter box, two plants is the smart, realistic choice. If you choose very compact varieties and stay on top of watering and feeding, three plants can work—but only if the box is closer to 30–36 inches long and the plants are genuinely dwarf. Avoid the temptation to fill every inch at planting time. Peppers grow wider than they look in nursery pots, and good airflow helps prevent disease in urban balconies and patios.
Choose varieties suited to containers, such as compact bell peppers, shishitos, jalapeños, or dwarf ornamental peppers. Use a free-draining, compost-rich potting mix, place the box where peppers get 6–8 hours of direct sun, and water consistently. Add support for heavier varieties, and feed lightly once flowering begins. If you are unsure, plant two. Two healthy, well-spaced pepper plants will almost always outperform three crowded ones in a medium urban planter box.