Anchor bolt and anchor cable are the most commonly used supporting measures in engineering slope treatment and underground engineering excavation. Monitoring instruments are arranged at a certain number of anchor bolts and cables to reflect the stress condition of support. However, the requirements for the number and proportion of anchor rods and cables differ in four relevant specifications in China. We made a research on the monitoring arrangement of ten representative large-scale water conservancy and hydropower projects, and gave reasonable suggestions on the number of monitoring instruments via mathematical statistical calculation and analysis. Our findings suggest that the representativeness of monitoring results of bolt and cable is closely related to the number of samples, but not directly related to the total number of samples and the sampling proportion; the heterogeneity of the monitored samples has a significant impact on the sampling representativeness; when the sampling number is less than 10, the sampling error drops significantly and the sampling representativeness rises significantly with the increase of the sampling number; when the sampling number is greater than 50, the increment of sample representativeness is not obvious. In conclusion, when large-scale anchor bolt and cable support is used, the monitoring should be arranged in line with local geological conditions and engineering structure rather than uniform monitoring proportion.