Can I change the number of necks on the round bottom flask?

Yes! By default, the round bottom flask has 3 necks.

In the property panel, select the number of necks as needed.

If an even number of necks is selected and one of the necks is not on the correct side, click the "Flip horizontally" button.

Note that similar to other glassware, tubes and rubber hoses will snap to the necks, if the angle is close to that of the neck.