That's an excellent question, and one that plaza managers and urban designers think about a lot. For a small urban plaza, there's no one-size-fits-all number, but a great rule of thumb is to start with one bin for every 30-50 feet of seating or high-traffic area, and a minimum of 2-3 bins total even for a very compact space.
The goal is to make disposal convenient without cluttering the landscape. Think about "points of generation": place bins near seating areas, food vendor spots (if any), entrances/exits, and pedestrian pathways. People won't walk more than 30 feet to throw something away. Using a mix of 1-2 larger capacity bins for main areas and a couple of smaller, more discreet ones for secondary spots often works well.
Material matters too. Durable, weather-resistant, and preferably anchored bins prevent mess and theft. Crucially, the number of bins is directly tied to your maintenance schedule. More frequent emptying (daily, even twice a day in busy areas) is far more important than having dozens of overflowing bins. Start with 3 well-placed, regularly serviced bins, observe where litter accumulates, and adjust. A clean plaza invites people to stay and return.