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This paper is concerned with the problem of exploiting spatial multiplexing gain through opportunistic beamforming with multiple random beams. The base station transmits multiple pilot sequences using orthogonal beams, and each user feeds the channel gain of each beam back. Then, the base station selects the subset of users and beams for which the sum-rate is maximized. We aim to exploit spatial multiplexing gain in a controlled fashion, i.e., the number of beams used for data transmission is chosen depending on the available multiuser diversity in the system. Two schemes are proposed: dynamic Orthonormal Random Beamforming with Systematic Beam Selection (ORBF/SBS), for which the number of beams used for data transmission is dynamically changed from one fading block to another so that the sum-rate is maximized; and static ORBF/SBS, which selects a fixed number of beams for a given signal-to-noise ratio and user number. The static scheme achieves the sum-rate of the dynamic one up to a small gap with much reduced computational and feedback complexity. We derive an approximate expression for the sum-rate achieved by the dynamic scheme, and provide insight into the optimal data stream number in the static scheme by means of asymptotical results. Computer simulations are provided to evaluate the performance of the proposed schemes.