What is the limit of deducting HELOC interest on a Rental Property in 2018?

For your 2017 taxes, if you have a 1098 mortgage interest statement, claim it. All that mortgage interest stuff doesn't kick in until you file your 2018 taxes next year.
For 2018 Business property mortgage interest fully deductible. primary residence mortgage interest deductible on the first $750,000 of your outstanding mortgage balance.

If the HELOC was for rental expenses (such as improving the home), the interest is fully deductible, regardless of the amount.

If it was used for non-rental expenses, the interest might not be deductible at all.  If you are unsure, please explain what the money was used for.