Writing unit test with JUnit and Mockito

I have a method in service layer for saving an item to database. This method first checks if there is an item with the same name in database. If there is, then it throws an exception with a message, if there is not then it just saves the new item. Here is the code;

public TodoList saveTodoList(TodoList todoList) {
    List<TodoList> match = StreamSupport.stream(getAllTodoLists().spliterator(), false)
            .filter(list -> list.getName().equals(todoList.getName()))
            .collect(Collectors.toList());

    if (match.size() > 0)
        throw new ExistingDataException("Todo List already exists!");
    else
        return todoListRepository.save(todoList);
}

I am trying to learn testing with JUnit 5 and Mockito and I am a bit confused about what to test in general. How many tests should I write for this method? Do I need to test stream(I don't even know how)? How can you decide what to test in a method?

Edit: Here is my test class and test methods for the save method;

@ExtendWith(MockitoExtension.class)
class TodoListServiceTest {
    @Mock
    private ITodoListRepository todoListRepository;

    @InjectMocks
    private TodoListService todoListService;

    @Test
    @DisplayName("Should save the list to the repository")
    public void whenGivenTodoListDoesNotExistInDatabase_ThenSaveNewList() {
        TodoList mockList = mock(TodoList.class);
        mockList.setId(1);
        mockList.setName("List 1");

        when(todoListRepository.save(any(TodoList.class))).thenReturn(mockList);

        TodoList actual = todoListService.saveTodoList(mockList);

        assertEquals(mockList.getId(), actual.getId());
        assertEquals(mockList.getName(), actual.getName());
    }

    @Test
    @DisplayName("Should throw exception if there is a list with given item's name")
    public void shouldThrowExistingDataException_IfThereIsAList_WithTheSameName() {
        TodoList mockList = mock(TodoList.class);
        mockList.setId(1);
        mockList.setName("List 1");

        Throwable actualException = assertThrows(ExistingDataException.class,
        () -> {
            todoListService.saveTodoList(mockList);
        });

        assertTrue(actualException.getMessage().contains("Todo List already exists!"));
    }