Unfortunately, once a part model has been created in KE-chain, the users will be unable to change its quantity. The only solution is to delete the respective part model and re-create it with the desired quantity. This means you will have to rebuild all the properties and part models under the deleted part model, as well as the part instances created based on it, so it is preferable that you spend enough time in thinking how you will organize your data model at the beginning of a project.