I have sent a Verpflichtungserklärung to my mother in Sri Lanka to invite her to Germany for a couple of months. In the check list for documents, financial proofs and financial statements are listed. Is there a minimum number that she has to show to get through? How do they evaluate it? Any advice on this would be much appreciated as I couldn't find much online.
The Schengen area requires that visitors show how they plan to pay for their subsistence in the Schengen area and the travel out of the Schengen area. Germany assumes that subsistence will cost about €45/day for a tourist.
Your mother will show that her subsistence is provided by you, so she will not need quite as much cash or bank balance. It still makes sense for her to have some money or credit during the trip, enough for a night in a hotel and a lengthy taxi trip, just in case there is any disruption of travel.