for online selling

for online selling registration is mandatory there is no threshold limit for online business

No need to take registration till your aggregate turnover exceeds 20lac

But .. note.. URD cannot sell interstate , so if doing so, then have take registration compulsory irrespective of your turnover

If turn over is below 10 lakhs, no gst registration required but if inter-state supplies made then GST registration is mandatory.

can i sell many different hsn code goods

No problem go ahead...

thnx sir

you need gstin without it you not able to sell goods online.

Threshold limit for registration also applicable to suppliers using e-commerce platform

SECTION 23, READ WITH SECTION 9, OF THE CENTRAL GOODS AND SERVICES TAX ACT, 2017 - REGISTRATION - PERSONS NOT LIABLE FOR - NOTIFIED PERSONS

NOTIFICATION NO.65/2017-CENTRAL TAX, DATED 15-11-2017

In exercise of the powers conferred by sub-section (2) of section 23 of the Central Goods and Services Tax Act, 2017 (12 of 2017) (hereafter in this notification referred to as the said Act), the Central Government, on the recommendations of the Council, hereby specifies the persons making supplies of services, other than supplies specified under sub-section (5) of section 9 of the said Act through an electronic commerce operator who is required to collect tax at source under section 52 of the said Act, and having an aggregate turnover, to be computed on all India basis, not exceeding an amount of twenty lakh rupees in a financial year, as the category of persons exempted from obtaining registration under the said Act:

Provided that the aggregate value of such supplies, to be computed on all India basis, should not exceed an amount of ten lakh rupees in case of "special category States" as specified in sub-clause (g) of clause (4) of article 279A of the Constitution, other than the State of Jammu and Kashmir.