How can fill "Fair rented days" and "Personal use days", if the rental is a multi family property, and has different vacancy days?

Rental Days - The number of days the property was rented. Weather there was a renter physically occupying the property is irrelevant. If you were paid rent for 90 days, then the property was rented for 90 days.

Personal use days - The number of days that you used the designated physical portion  of the property classified as a rental for any type of personal use from the day the property was classified as a rental. What you used the property for before you converted it to a rental does not count. (read the "Note" on that screen.)Typically, this number will be a zero.

If there was personal use, you should treat it as separate properties.