Currently there is no way that I can think of to restrict number of entries the user can add per view or per field content for that matter.
You can work around that, by adding the "project" entries on behalf of the students. You can bulk import such entries via a csv view. Students will enter the project description in these entries. All the other entries they will add in another view will be the activity entries. So you don't have to limit the number of entries in the activity level.
hth