7 Where individual dioceses are unable to institute their own seminaries properly, seminaries for many dioceses or for an entire region or for a country are to be set up and developed, so that the sound training of the students, which must be considered the supreme law in this matter, can be taken care of in a more effective manner. These seminaries, if they are regional or national, are to be regulated according to directives set down by the bishops concerned and approved by the Apostolic See. In these seminaries, however, where there are many students, while retaining a unity of direction and of scientific training, the students should be conveniently divided into smaller groups so that a better provision is had for the personal formation of each.
Source: Optatam Totius (Vatican.va)