If you are doing a series, you want there to be (partially) unanswered questions still open at the end of each book, along with the promise that at least some of those answers will be supplied in the next book. The trick is to make each book worthwhile in its own right. That is, the purchase and consumption of the book was not a waste of time for the reader.
On the other hand, you do not want to surprise the reader with material, characters, and plot in the later instances of the series that were not at least foreshadowed in earlier instances. For example, cue the super-powerful villain who has committed evil for decades, represents the protagonist's antagonist in the latest book, and was not even hinted at in any of the earlier books. Especially, if it is obvious that the writer needed to dig their way out of a plot hole.
The ideal here would be to establish a rich context for the story in terms of economics, law, politics, culture, science, technology and make it clear that the story told in each book is just one of many stories taking place against that context. The context does not have to be super detailed. For example, we introduce our villain of the moment, a proud graduate of the prestigious University of Villainy, and current treasurer of The Club of Nastiness. And before you exhaust yourself, hating this villain, note that, if this villain stumbles (or is pushed) off the cliff, the economic and historical situation of our story is such that another villain of much the same demeanor will be forced into existence.
If you are honest with your readers and they understand that this is a series and each book will provide some, but not all, of the story, and the part that you do provide is worthwhile, I think that you will be OK.
