How many genes have been associated with alzheimer's disease?

3 to date. Among the many genes and environmental factors that cause alzheimer disease, 3 genes have been associated with early age onset and mendelian (single gene) inheritance. See http://www.Mayoclinic.Com/health/alzheimers-genes/az.
Four. Three genes on chromosomes 1, 14, and 21 cause autosomal dominant inheritance (that means that 50% of children will get the disease), the fourth one, for apolipoprotein e, is just increasing the risk of getting alzheimer's disease at an earlier age.