Sail numbers for dinghies generally come in two sizes 300mm or 230mm. The 230mm sail numbers are suitable for small dinghies under 3.5 metres and the 300mm sail numbers for dinghies above 3.5 metres.
When changing sails, i.e. buying a second hand sail, it will be necessary to change your sail numbers if you are going to race. With some classes like lasers there are so many, you may never see the boat with teh same numbers, but other classes where there are less boats like Merlin Rockets or Solo, you will need to change them.
Changing sail numbers isn’t that difficult, but you may need some adhesive remover.
You can get quality yet cheap sail numbers from SailSport.