There is no set number – beyond the requirement for at least two independent measures – it should be the number needed to achieve the Appropriate Level of Protection in relation to the estimated risk. You may consider whether more than one of the harmonized list of objectives for measures is covered, but even this is not required.
The DSSA is able to accommodate up to 20 measures once you select those for further evaluation (Tables B2 and section C). If you have more, you may combine some similar ones or complete another file with a copied file, to cover those that make up a separate component of the system. For example, if there are a number of steps for registration of orchards and it is important to discuss all of them, you could make one file for those measures and start the next file with the measures for production stage onwards.