Tax on Long Term Capital Gains

I know that there is 0% tax rate on Long term capital gains if your taxable income is at or below a certain rate. I have two questions that I cannot find answers for anywhere.

First, What is considered your "taxable income" in this case? There are three values for this on form 1040SR  (Line 6b - Taxable Income, Line 11 - Adusted Gross Income, and Line 15 - Taxable Income, I.e. after all deductions)? 

Second, is there a limit on how much long term capital gains you can get tax free if you qualify via income? For example, can you go as high as no taxes on over $100,000 in long term capital gains?

Thanks