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If you're an absolute beginner, literally any keyboard will work (I'd check Goodwill). They usually come in 3 sizes: 61 keys, 76 keys, and 88 keys. The bigger the better, for longevity sake. Eventually (after several years) you'll need all 88 keys and it will be good if they are kind of hard to press down, like a real piano ("weighted" keys). But those are costly upgrades (semi-weighted, 88 keys are in the $500-$800 range new) and a LOT can be learned and practiced on a cheap, used keyboard. It's not what you play on, it's how you play it. If you're wanting to buy new, here are a few examples of appropriately-priced keyboards:
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