How do I calculate "Fair rented days" for my rented property>?

You would count the  number of days that you rented your property for the Fair market rental value.  Do not count days that you used it for personal use.

Rental property / personal use

"If you rent a dwelling unit to others that you also use as a residence, limitations may apply to the rental expenses you can deduct. You're considered to use a dwelling unit as a residence if you use it for personal purposes during the tax year for a number of days that’s more than the greater of:

1. 14 days, or
2. 10% of the total days you rent it to others at a fair rental price.

It's possible that you'll use more than one dwelling unit as a residence during the year. For example, if you live in your main home for 11 months, your home is a dwelling unit used as a residence. If you live in your vacation home for the other 30 days of the year, your vacation home is also a dwelling unit used as a residence unless you rent your vacation home to others at a fair rental value for more than 300 days during the year in this example.

A day of personal use of a dwelling unit is any day that the unit is used by:

• You or any other person who has an interest in it, unless you rent your interest to another owner as their main home and the other owner pays a fair rental price under a shared equity financing agreement
• A member of your family or of a family of any other person who has an interest in it, unless the family member uses it as their main home and pays a fair rental price
• Anyone under an agreement that lets you use some other dwelling unit
• Anyone at less than fair rental price"