I think there is a mix of concepts, defining each one of them well can help to find a solution.
Sketching is part of one of the first phases in a design, the brainstorming's graphic result. In this step there's not a simple sketch or a concrete definition of the project, but rather there are many ideas embodied in a support destined to find a final concept. In general, the first sketches not only looks for a graphic result but also a conceptual one.
While the prototype sketch belongs to a more advanced phase: the essential idea of the project or concept is already defined and these prototypes are schemes that only seek the practical way to place it into the final support.
Personally, I would not abuse showing sketches of the achievement of a project idea in a case study for the following reasons:
If it's a project for a personal portfolio, only three essential sketches of how that final graphic and conceptual idea was arrived would be enough
If it's a homework to show to a teacher, I would not include the original drafts in the final presentation unless this requires a discussion, in which case I would set up a separate panel with the key sketches (not all of them).
If it's a project for sale, as a professional you have reached a final idea after having gone through several sketches. From all these, you have chosen the one that best suits the project, based precisely on your experience and professionalism, actually the client pays for it. If you have a number of original sketches in the presentation of a project, you are contradicting yourself: a chef only mentions the main ingredients of the dish, not the recipe, much less how he/she came to it.
If it's a project to present to a client, one of those non-chosen
sketches may seem like a better idea to him/her. Among the different
client's "psychologies" is the one that "your idea is fine, but if
I can place something of mine much better." Putting countless
sketches on a project presentation discussion table is putting many fishes in the river of a hypothetical client's vanity, a very
negative factor for the sale. If this is the case, the weight of the client's choice will be above that sketch idea, which may lead you to have to rethink the entire project.
Very different is the case of prototype sketches, in this case they can be of great help to understand the project functionality. If this is the case, I would place the essentials, although graphically well presented.
There's a very good explanation about creative processes in careerfoundry.com (the image below) where is very well defined and differentiated the ideate stage and the prototype stage.