Do I have to create one Time-Off policy for vacation days per employee?
No, you must determine how many vacation policies are applicable within this employer, based on different number of vacation days per year.
So for example if you have employees with 15, 18 and 20 vacation days per year, you must create three different Time-Off policies and link the referring employees to this policies.
A part-time employee who does have 15, 18, or 20 days on a full-time basis does not need to create a separate plan for it but rather indicates it in the plan: Apply part-time percentage [YES}