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Date Night On Union Station. I got it off amazon. It's free and a good read. There is a race of sentient computers in it that have their fingers in trade and stuff like that. I guess there are 4 books total. LINK
I know you said fantasy but trust me try this it's science fiction but If you want some light feel good reading this is a great series https://www.amazon.com/Night-Union-Station-EarthCent-Ambassador-ebook/dp/B00K4I391A
First, in response to your question, it depends. Having a consistent theme or base art for a series can help with branding. Look at the Union Station novels by E.M. Foner for example. Those are full sized novels, not novellas, but you can see how they are instantly recognizable.
If they are each going to be novella length, I would use one cover as E.M. Foner did, otherwise your cover design costs will eclipse profits or at least cut deeply into them.
Now, as to an unasked question - is selling a series of novelettes like that the best strategy for your story? Have you heard of web serials? They are an interesting market, the authors usually publish on their own wordpress site and a genre portal or two. The works are freely available and benefit from the built-in audience of a given portal site. Revenue comes from paypal and Patreon donations. The top authors on Royal Road, for example, are making a very decent living, but they represent only a small portion of the authors on the site, of course.
Still, such a site is a perfect match for a serialized story release schedule, they have comments sections full of people offering both feedback, criticism, discussion, and editing suggestions. You could publish the entire story on such a site, get it to a good quality, and then publish on Kindle. If you wanted to publish on Kindle Unlimited, you would have to remove it from the free sites - your blog and the portal. The upshot of that though, is you could let the audience know the collected works are available as Kindle downloads at that point. They'd also be able to provide reviews on Amazon having already read the story. A few authors have done this.
and another one
What is your genre? There may be a site whose brand and set of genres would be a good match for your work.
For science fiction/fantasy and LitRPG in particular, check out Royal Road.
I know that's a big segue from your initial question, but it sounded like you might not be familiar with the web serial option, so I thought I'd mention it.
The EarthCent series by E.M Foner is pretty good. It's generally low stakes slice of life scifi.
Try EM Foner's Union Station series. Great fun and good reads.