How much money do I need to make to sponsor someone to get a green card?

My wife is from Shantou China and I am going to file the adjustment of status for her to get a green card. My question is about the financial sponsor. I know that I need to make at least $20,000, give or take. I am getting a little confused on the Household Size questions on the Affidavit of Support and the I-864P Poverty Guidelines. Please help. I have 2 daughters and now will be sponsoring my wife to get the green card. What can I do?