Question

My outward supply in Haryana is rs. 10 lakh and i voluntary taken the gst no. now i have started business in Delhi also having turnover rs. 7 lacs only total rs. 17 lacs only. whether I need new number in Delhi also under GST.

Answer

Facts of the Case: As per 22(1) since your aggregate turnover is not above 20Lakh you do not need any registration (assuming that you do not fall under section 24 of compulsory registration).
However you exercised your option under section 25(3) to get voluntary registration for haryana unit.

Law applicable: As per 25(3);
" A person, though not liable to be registered under section 22 or section 24 may get himself registered voluntarily, and all provisions of this Act, as are applicable to a registered person, shall apply to such person. "

Interpretation: Once a person gets voluntarily registered from unit under section 25(3), all the other provisions of the act shall apply normally to such person and there shall not be any difference for the purpose of this act between a voluntarily registered person and other taxable person registered under section 22 or 24.

Conclusion: The voluntarily registered person is now as good as other taxable registered person and shall therefore get registration of all his units from where taxable supply is made. Therefore in your case Delhi registration is required. (Reply dt.12/03/2019)