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This letter investigates the achievable capacity scaling of power-constrained amplify-and-forward (AF) opportunistic relaying (OR) for a large number of relays K with limited feedback. In order to reduce the feedback overhead, this letter adopts a selective ID feedback through contention-based feedback channels. Considering the feedback congestion error, this letter shows that the selective ID feedback closely achieves the capacity growth rate 1/2 log log K, and simultaneously reduces the feedback overhead by a large amount. An appropriate choice for the threshold, which can be calculated distributively at each relay, is also given.