GST No. Validity will be application date or the approval of Registration date.

Where the application for registration is submitted within 30 days of becoming liable to apply for registration (i.e. the date of crossing the threshold limit of ₹ 20lakh/40lakh), the effective date of registration is the date on which the person becomes liable to registration.
If a person fails to apply within 30days of crossing the threshold, the effective date of registration is the date of grant of registration certificate.

Further, every registered person who has been granted registration with effect from a date earlier than the date of issuance of registration certificate to him, may issue revised tax invoice in respect of taxable supplies effected during this period within 1 month from the date of issuance of registration certificate.(Section 31(3)(a) of CGST Act,2017 r/w rule 53(2))

In short:
In case you have crossed threshold and you have applied within 30 days of crossing threshold then you can create tax invoice but after getting GSTN you have to issue revised invoice with GST number alloted to you by authorities.

In case you have applied for voluntary registration than you can raise tax invoice only from date of grant of registration certificate

As per Section 31(3)(a) - a registered person may, within one month from the date of issuance of certificate of registration and in such manner as may be prescribed (Rule 53), issue a revised invoice against the invoice already issued during the period beginning with the effective date of registration till the date of issuance of certificate of registration to him;

 

Rule 53(2) - Every registered person who has been granted registration with effect from a date earlier than the date of issuance of certificate of registration to him, may issue revised tax invoices in respect of taxable supplies effected during the period starting from the effective date of registration till the date of the issuance of the certificate of registration:
Provided that the registered person may issue a consolidated revised tax invoice in respect of all taxable supplies made to a recipient who is not registered under the Act during such period:

 

Example: in Form REG-06 - Effective date of registration is 10th May 2020, although the registration certification was issued on 30th May 2020, then for such period, invoices can be raised with GST and after receiving the certificate, revised invoices with GSTIN could be raised.