A scalable security system is designed so cameras, doors and storage can be added without replacing the core. That means spare switch ports and power budget, recorder channel and license headroom, documented addressing, and a management platform that supports more sites or users.
Growth exposes whichever limit was ignored at installation. Expansion is cheapest when the original design left room for it, and the first constraint is rarely the camera count printed on the box; it is usually power, uplink capacity or storage.
The limits that appear first
Switch power budget, uplink throughput, recorder throughput and disk space all bind before the channel count does. Check each against the expected final size, not only the opening phase.
Designing for the next phase
Leave rack space, address ranges and conduit capacity for later work, and document the design so another technician can extend it. Documented headroom is the only kind that gets used.