For me I think the most dreams I've remembered in one night was 8. That only happened once though. And that's not including lucid dreams. Anyway, for me a dream is defined by it's scene and plot. If the scenery changes but the plot and characters stay the same, then it's the same dream. But if the scene changes and you are doing something different with different people then it's a different dream. This is just my own opinion I'm sure everyone has their own opinion about what constitutes a seperate dream for them. Usually you have a good idea about when a new dream begins because the other dream either fades out, becomes blurry, or you just don't exactly remember how it ended.