Smallest ITX case with one expansion slot? (i.e. M350, etc) | SFF ...
With an ATX motherboard, it’s also easy to get 7-8 expansion slots.
Motherboard Expansion Slots - CompTIA A+ 220-1001 - 3.5
With more and more features integrated into the CPU and motherboard chipset, a large ATX motherboard with seven expansion slots is starting to look like overkill for most people.
Expansion card
The document discusses various components of a motherboard including the CPU, expansion slots, expansion cards, RAM slots, ports, and connectors.