Groundwater monitoring of an aquifer performed by an observation well network plays an important role in the management of groundwater resources in which, if the number of wells is small, the calculation of groundwater level is less accurate, and if the number of observation wells is high, the cost of drilling and maintenance of wells and the cost of measurement will increase. This study aimed to identify observation wells' number and optimal density in an aquifer using classical statistics and geostatistical techniques. The Sarfiroozabad aquifer with 45 observation wells was selected as a case study, in which the effective range of each observation well was determined through examination of the coefficients of water table variations in parallel profiles perpendicular to the longitudinal axis of the aquifer and examining geostatistical items including variograms related to water table variations by specifying the optimal number of observation wells. Classical statistics and geostatistics showed the same effective range for wells in the northern aquifer. According to the observations, to obtain the optimal number and density of observation wells, at least 6 observation wells shall be added to the northern aquifer, while only one well should be added to the southernmost part of the southern aquifer and six wells should be eliminated that reduces the cost of sampling. Also, by adding essential wells, the accuracy of groundwater estimation increases which helps better manage groundwater resources.