What are the chances for a 20 year old female to have lupus?

Nobody knows, but: Nobody knows the chance of a particular 20 year-old woman to have lupus. The estimates are 1.3 million women in the US have lupus. Patients are diagnosed usually in the 15-44 years-old range in US and UK. Charts suggest that each year, 1 of 15,000 women around age 20 get diagnosed with lupus (hopefully not your particular 20 year-old). Look at... https://www.ncbi.nlm.nih.gov/pubmed/19136143