You haven't provided enough info to answer that question.
As you appear to know, when you file an extension, you still need to pay the amount of tax due before April 18. Normally, you would be far enough along in your tax prep that TT has calculated a preliminary tax due. You would pay a little bit more than that to be safe.
If you haven't gotten that far, you can use this tool to do an estimate: https://turbotax.intuit.com/tax-tools/calculators/taxcaster/?s=1
