There's many ways to play a legacy game, it doesn't have to specifically be "10 generations" and you could start with more than one sim even, provided they are all part of a single household. There is, of course, the official unofficial "legacy challenge" ruleset, but a game can be a legacy playstyle without someone following the challenge rules. You will find some simmers that tell you that "you aren't playing legacy" if you don't follow the challenge ruleset...they are just being snobs.
The term legacy as a Sims playstyle is a reference to the 2nd definition of the word "legacy" as given by dictionary.com
Legacy: anything handed down from the past, as from an ancestor or predecessor:
So any game where you follow a bloodline, specifically playing whichever descendant(s) inherit the fruits of their predecessor's efforts, would make you a legacy player. Your style of playing multiple households wouldn't be "legacy" though, as it is more than one bloodline. You could use the term "Legacies" (plural) if you wanted though assuming you are following some form of inheritance system with each bloodline. :)
If you are actively playing more sims than those which actually get anything (be it money, the house, objects, etc.) from the previous generation though, you start to fall outside of legacy play.
