Mortgages 9over $1 Million) have tough rules. The law allows interest deductions on up to $1 Million of “acquisition debt”, lus another $10,000 of “equity debt” for your main home, plus one other property you use personally.
“Acquisition” debts are to “buy, build, or improve” the property.  “Equity” debt is any other debt secured by the property.
Note:  It is my understanding that IRS is now routinely auditing any return claiming over $75,000 in mortgage interest.