Within a KNX area, the maximum number of Line Couplers is 15, one per subsidiary line.

True. A KNX area is built around one main line that connects up to 15 subsidiary lines, each attached to the main line through its own Line Coupler. The limit of 15 follows directly from the topology addressing scheme, which reserves 4 bits for the line identifier within an area. Each subsidiary line itself can then host up to 64 devices (or 256 with additional line segments and segment repeaters).

When the project approaches the 15-line ceiling per area, start planning the next area in advance — adding a sixteenth subsidiary line later means a topology rework, not just a wiring change.