19" racks and rack units
Servers don't sit on a shelf — they live in tall metal cabinets called racks. Each rack is divided top to bottom into slots called "U." A thin 1U server takes one slot; a chunkier 2U server takes two. The taller the rack, the more gear it holds, so rack height is the first ceiling on how many machines you can run in one spot.
A rack has a height limit AND a power limit, and you can hit either one first. Your garage starter rack is short (12U) and on a thin power circuit, so you may run out of watts long before you run out of slots. Pack a rack with the math in mind — empty U above a maxed-out breaker is wasted floor space you're still paying rent on.
Detailed explanation
A standard 19" rack is 42U tall (about 1.87 m); one U is 44.45 mm (1.75 in). Hosts, switches, and patch panels all consume U off the same per-rack budget — there is no separate switch chassis. The starter garage rack is 12U on a 3 kW power budget; standard datacenter racks run 42U at roughly 8 kW.
Slot capacity (slot_capacity) and the rack power budget (power_budget_w) are independent constraints — whichever exhausts first caps the rack. A dense compute fleet can strand free U because the breaker is the binding limit; a storage-heavy rack can strand watts because it runs out of U. Plan placement against both, not just the one you happen to be watching.