Why would you ever create one dashboard with that many sheets? That is not really usable for end users.
You should break them up into separate apps with maybe 6-10 sheets per app. Even that is a lot in my opinion.
If you have to do it I don't think that there is a way out of the box to group sheets into sections or folders like you want to. The user will just see 100 thumbnails when they open the app. You could name the ones you want together with similar names so that they make sense visually side by side.