A nationwide survey of help-wanted advertising indicated that demand for workers declined in November, probably because of the Oct. 19 stock market crash, the Conference Board, a business information group reported. Its seasonally adjusted help-wanted index stood at 160 last month, down from 163 in October but still up substantially from the 143 recorded in November, 1986. The index measures the volume of classified advertising in 51 major newspapers. Based on a scale of 100 set in 1967, it reflects the strength of business demand for labor and is sensitive to general business conditions. A decline of 7.6% was registered in the Pacific region.