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For multiple-input multiple-output wireless systems, we investigate the important question of how many antennas to place in fixed-length arrays, accounting for both spatial correlation and mutual coupling at the transmitter and receiver. We show that there is an optimal antenna configuration yielding the highest capacity which depends strongly on the array length and the transmission wavelength, but does not depend strongly on the signal to noise ratio (SNR). Moreover, we show that ignoring the effect of mutual coupling gives misleading results, yielding unbounded capacity growth. As another key finding, we demonstrate the surprising result that if the optimal antenna configuration is employed, then further optimizing the transmission based on the channel statistics gives very little benefit over simple equal-power spatial multiplexing. By deriving an expression for the capacity at low SNR, we provide a straightforward method for estimating the optimal number of antennas.