Title 14 chap 246 sec. 14-18
(c) Such number plates when displayed upon motor vehicles shall be entirely unobscured and the numerals and letters thereon shall be plainly legible at all times. Such number plates shall be horizontal, and shall be fastened so as not to swing and, during the time when a motor vehicle is required to display lights, the rear number plate shall be so illuminated as to be legible at a distance of fifty feet. No plates, devices or attachments may be affixed to or covering the official number plates. Not more than one number plate shall be displayed on the front or rear of any motor vehicle in operation upon the public highways of the state; provided any motor vehicle may, upon permission of the commissioner, display more than one number