Hi @Mericki44,
In my opinion, the best way to handle this would be to grade everything (including giving missing/unsubmitted assignments as a 0, using missing policies if you'd like). If you do things this way, you can set up an assignment group rule to drop the lowest X number of assignments. In your case, since you want to keep 5, you'd set X to be the number of assignments in the group minus 5. This does assume that all assignments in the group are worth the same number of points. If they are varied points, the method will still work, but each student's total score may end up being out of a varied number of points possible.
Hope this helps!
-Chris