L3 cache used to be a difference between lower end CPUs, which had no L3 cache, and the more capable but more expensive mainstream CPUs. Now it's fairly common to see most CPUs have it.
How much they have is a difference now. In the case of streaming, no there is not a certain amount you need. For streaming you'll want cores and threads. You'll need enough cores to run the game, then one or two extra to run the streaming software. So if you were planning to stream a game that you'd normally need a quad core to run, then you'd want a six core CPU to game and stream, or maybe a quad core with hyperthreading.
Dota 2 is not that demanding so maybe a quad core would be enough for streaming that game, unless it was an old quad core.