Even if you are an unregistered person, you need to maintain copy of invoices for verification purpose.
Your invoices are proof that your turnover has not crossed 20 lakh Rs.
For second question, if you are unregistered it makes no difference whether you write GST number or not.
If you are registered the person will not get input credit if you write wrong details.
You can read full details here ( GST invoicing rules ).
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