One billboard can work. Multiple billboards usually work better.
The reason is repetition. Customers rarely act the first time they see an ad. They act when they recognize the business, remember the name, and need the service. Multiple billboards in the same market can create that repeated exposure.
For a small local business, one excellent billboard may be enough to start. But if the goal is to dominate a market, multiple boards can help cover different traffic patterns. One board may reach morning commuters. Another may reach shoppers. Another may reach interstate traffic. Another may reach people near the business location.
A good market plan may include:
The right number depends on budget, market size, and campaign goals. A restaurant may want directional boards near exits. A roofing company may want broad local coverage. A medical clinic may want boards near neighborhoods and retail traffic.
A billboard campaign should not be random. It should create a net around the customers the business wants most.
Best rule: one board creates awareness; several boards create market presence.