Tax is applicable on Long Term Capital Gains (LTCG) if your income (NOT INCLUDING LTCG NOR DIVIDENDS) exceeds
Head of Household $55,800
Married Filing Joint $83,350
So, amounts below those have no long term capital gains.
The LTCG rates are 0%, 15%, or 20%. And then another 3.8% might apply depending on income.(It is called Net Investment Income).
Also, it is somewhat deceiving.
Adjusted Gross Income includes LTCG and could cause a portion of social security to be taxed. Also, some states tax LTCG as ordinary.