That's the initial cap on 2 personal residences. mortgage interest on a third is not deductible but you get to choose which 2.
however, there is wiggle room. IRS pub 936
say you buy a home in 2018 with a 500K mortgage. you buy a second home in 2019 with a mortgage of 270K. the average balance on both for 2019 is 750K. all interest is deductible because the average balance of the acquisition indebtedness for 2019 does not exceed 750K.
