Single Niche or Not?

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Well, I am trying to make a living online and I decided to have a blog. I would just like to know whether it makes a whole lot of difference if my blog only has one niche or not. Would I earn more if I focused on one topic? Would advertisers prefer one niche blogs over a random niche blog? Is a personal blog better than an informative blog? I would also want to know tips and possible topics for a blog to earn more. Thanks.