The sail number for a wooden boat is engraved or stamped onto the inside of the (aft) transom doubler on the starboard side (right hand side looking forward). The number will be in Arabic numerals a minimum of 20 mm high. The transom doubler is the piece of wood which makes the visible part of the aft transom (i.e. above deck level) thicker than the plywood used for the rest of the hull so that it is strong enough to mount the top rudder fitting, the mainsheet attachement points and a small outboard motor.
The hardwood species used for the transom doubler can vary (mahogany, obeche,..) and even within a species, timber will vary, so kits from manufacturers who stamped the number on (i.e. Bell Woodworking) may vary in how visible the number is.