In general, for 2D positioning, the minimum number of anchors needed is 3. However, we recommend 4 anchors for additional reliability. For 3D positioning, at least 6 anchors are required.
With 4 anchors you can cover about 50×50 m (~2500 m2) in ideal conditions. Ideal conditions almost never occur in real-life. For industrial applications on average 4 anchors can cover 500-1000 m2. So a grid of between 20×20 and 35×35 is common.
The number of anchors you need to cover the tracking area heavily depends on the environment and the use case. The environment means how many obstacles are in the area for radio performance. The use case determines the required coordinate accuracy & reliability. Providing check-point tracking requires much less anchors compared to safety applications.
Contact us to discuss your use case and get a preliminary quote