I'm a qualified Chartered Accountant and would try to answer your queries here, so that anyone else who has similar queries can find it useful.
1. I have to take GST number ?
--> You are required to apply for GST registration if your turnover in a financial year exceeds INR 20 lakhs. However certain international payment gateways are asking for GSTIN for international transfers irrespective of your turnover threshold. Thus, if you are planning to receive from these payment gateways, you shall be asked to furnish a GSTIN
2. How to show this payment is by income tax book ?
--> The receipts will be your income and you need to show it as revenue from services. You can claim expenses such as office rent, electricity bill, Telephone/WiFi bills, etc. However if you are an individual with turnover less than 50 lacs, you can opt for Section 44ADA and claim 50% rebate in your income tax filing without maintaining any books of accounts.
3. I have to print a bill against 'upwork inc' for every payment received ?
--> No, the invoice that upwork is generating on your behalf is sufficient documentation, you are not required to re-do the invoices again.
Hope my answers are helpful, if you have further doubts, you can send me a DM here. Thanks!