Whatever it needs to be for your story and your audience.
A lot of Agatha Christie’s novels seem to land somewhere between 40k and 60k words. A lot of general fiction nowadays seems to be around 80k-90k.
A lot of classics soar wayyyyyy past 100k.
The Mouse and the Motorcycle was under 23k words.
Charlie and the Chocolate Factory was about 31k.
The Lion, the Witch, and the Wardrobe was 36k.
Black Beauty and Lord of the Flies were around 60k words.
Treasure Island was around 67k words.
The first Harry Potter book was about 77k words.
Anne of Green Gables was 97k.
So-- get it down on paper first. Worry about plot and plot twists; worry about characters and dialogue; worry about all the internal stuff that you need to worry about. Presumably, you’re already familiar with the requirements of the publishers in your genre, and your readers’ expectations, and all that good stuff.