Hiring one big celebrity vs hiring multiple

Not much information provided with your question, so just going to speak very generally. In general, a bigger celebrity is going to be great for branding and getting people to notice your brand. Essentially getting a lot of exposure, but with additional credibility as its tied to that person. With that being said, that doesn't mean that this exposure will necessarily convert to sales.

I think its a question of finding celebrities that have strong engagement within a niche but may not reach millions of people vs a celebrity that reaches millions of people but doesn't cater to any specific niche.

In general, it all just depends on your brand and product, who your trying to target, what your goal is (exposure, sales, etc), and of course what you can afford.