Max of 4 properties on a residential conventional loan30 year is best for residential Commerical will be a 5 year adjustable rate We don't know any rules of thumbDo an IRR calculationUse NerdWallet's or Bankrate's Mortgage Breakeven calculators https://www.bankrate.com/calculators/mortgages/mortgage-refinance-break-even-calculator.aspxKeep the psychological benefit of not having debt in mind