There are two separate questions, first the number of data locations to use in estimating/modeling the variogram and secondly the number of data locations to use in the kriging equations to interpolate the value at a non-data location (or to estimate the average value over a region). Assuming you are using a moving search neighborhood, more than 15-20 data locations in the neighborhood will likely degrade the results because (1) only the nearest data locations in the search neighborhood will have non-zero weights, (2) with more data locations the size of the matrix to be inverted is larger and the possibility of a an ill-conditioned matrix increase. The total number of data locations needed for kriging is dependent on the number of locations to be interpolated and the spatial patterns of those points and also of the data locations. In short, there is no simple answer to your question.
With respect to estimating/modeling the variogram it is a very different problem, see for example
1991, Myers,D.E., On Variogram Estimation in Proceedings of the First Inter. Conf. Stat. Comp., Cesme, Turkey,
30 Mar.-2 April 1987, Vol II, American Sciences Press, 261-281
1987, A. Warrick and D.E. Myers, Optimization of Sampling Locations for Variogram Calculations Water Resources Research 23, 496-500
These can be downloaded at www.u.arizona.edu/~donaldm