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You'd really need something in the order of about 30 watts from a valve amp if you wanted to be able to compete with a drummer and maintain a clean-ish sound without help from the PA and monitor speakers.
If you don't mind some distortion (or even a lot!) you probably don't need quite that much power, and more speakers will produce more volume- a 2x12" amp will probably seem louder than a 1x12" amp with the same power rating. Of course, more speakers means a bigger cabinet, and the two generally add up to a more expensive amp.
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