Start with a general store and test a bunch of products. Whenever you discover any hot-selling or high-in-demand product in your general store, go niche on that product by creating a separate account/website for it. The data you have gotten from your general store will help you scale the product faster in your niche store.
Keep testing more products in your general store and keep going niche on the few ones that are selling hot. If you’re into drop shipping or promoting a non-branded product, it’s advisable to keep your niche stores as private as possible. Your competitors will definitely see your ads and your store but they won’t know who’s behind it.
If you have been in the business for some time and have discovered your niche, you can focus on your niche.