The following instructions are added to the Instructions for Form 1098 for tax year 2017.
Additional instructions for Box 9: If an APN or other such identifying number needed to complete box 9 is not readily available for the property (having no address) securing the mortgage, enter a description of the property, using metes and bounds if available, or other descriptive language, to properly identify the property. You may abbreviate as necessary.
Additional instructions for Box10: If only one property secures the mortgage, you may leave this box blank. For purposes of determining the number of properties, you may consider any single physical street address, 911 address, lot, parcel, APN, or tract of land to be one property. If there is more than one property securing the mortgage, you may report the address of any one of the properties using boxes 8-9 and enter in box 10 the total number of properties securing the mortgage."