Gst registration - super urgent

Hey guys, I am working as a consultant and so far I have invoiced 19 lakhs ..every month I do invoicing of 2 lakh from 1 single client.....now, since I am applicable for GST when I cross 20 lakhs, how much GST should I charge my client...do I need to raise 2 invoices i.e 1 lakh each or 1 invoice of 2 lakh. Also, I am becoming an employee from February so I have to register for GST only for this invoicing. Please let me know what should I do....it's a bit urgent