Do I Need More Than One SBI
Site To Cover My Niche?

Question - Should I include several related topics on one website or build multiple sites instead to focus them better?

Answer - You could include all of your topics related to say, health and wellness, or maybe motor car racing, or whatever, on one site as long as they are all inter-related.

The search engines rank pages, not websites, so as long as your topics are somewhat related (the more related they are, the better), your site should do well.

I'd recommend one site to start with, then you could maybe branch out to get more specific on certain topics later when you see the demand growing.