Ok, this IS tricky because there are different scalability axis.
- How many computers you can happily have on a segment
- How much data you can move on a segment
- How much data you can move on a cable.
All those mean differen thigns and different ethernet level technologies migitate some of them more or less (switchesvs. hubs for one and two for example).
I would assume the most likely answer is network speed, which would be indicated by the different ethernet speeds (10megbit, 100megabit, 1 gigabit and 10gigabit).
Then we have whole network scalability. For example a switch with 48 ports - how scalable is it? These days mostly a non issue (answer: 96gigabit, the switch can hadnle the maximum theoretical traffic), but in the past a lower priced switch may not have enough processing power to handle this under all circumstances. As Ethernet is not necessarily an internet technology, requirements for speed and latency are stricter. For example I am using ethernet (like most people) between application servers and database servers, and there I push a LOT more bandwidth than most web servers do ;)