Yes, it is days actually rented (not days available for rent).
That is for line 2 of Schedule E, and the Instructions say "report the number of days in the year each property was rented at fair rental value".
If there is no personal use, it is fairly common to just enter 365 rental days even if there was some vacancies because the days really doesn't matter when there is no personal use. It is technically incorrect to do that, but because it doesn't affect anything, there is no harm done for doing that.
If your accountant has been using the days AVAILABLE to rent when you also used the property for personal use, you have the opposite problem of what you had thought. You have been claiming TOO MUCH expenses and too much depreciation for those years.
