The Altair 8800 had a backplane with 18 connectors for S-100 cards.
To start with, the genuine Altair had a 4 slot backplane, or 'Expander Board' as they called it. To cite their advertizement:
The basic Altair 8800 includes the CPU, front panel control board, front panel lights and switches, power supply and expander board (with room for 3 extra cards) all enclosed in a handsome, aluminium case.
If anyone needed more slots, multiple 'expander boards' could be mounted side by side and connected by soldering 100 wires.
When IMSAI introduced their Altair clone with a 22 slot backplane, the wire-extended multiple-of-4 solution looked rather clumsy in comparison. MITS responded with their B-model now offering 18 slot.
Why was the number 18 particularly chosen?
For the space available?
The question might be rather why the 8800b not had 22 like the IMSAI 8080. The reason is the way the Altair power supply was placed. IMSAI did design their PS to occupy the right side of the case, so the backplane could be placed along (front to back), giving space for 22. In contrast MITS' design placed the PS at the rear. Without changing the PS it was only possible to put an extended backplane across (left to right) of its 19" enclosure. So 18 it was (*1).
Space wise the maximum would be around 35 with tighter packaging of slots and no oversized cards.
It is fairly common for a computer to have the number of expansion cards be a power of 2, or some number slightly less than a power of 2.
Any source for such a generic assumption?
Often this is a result of how the address space is divided. But this is slightly more than a power of 2, so there is likely some other reason.
Address decoding is exactly not the point here. The S100 bus is for most parts simply a collection of all decoded 8080 CPU signals. It does not provide any services, like address decoding, not does it add any limitations, beside what the CPU provides.
Altair Bus in particular and S100 in general can have any number of slots/boards. Anything from a single slot to >20 has been produced. Of course overly high numbers will add issues in terms of timing, power and signal quality. Thus the IEEE 696 later introduced a maximum path length (on the backplane) of 25 inches and a maximum number of 22 slots.
*1 - It might be noteworthy that the Altair didn't use a case made to the 19" standard, but a custom (read cheap) one that would fit onto a shelf inside a 19" rack. Outer dimensions were 17 (width) x 7 (height) x 18 (depth).