Most of them. The ones that uses Proof-of-work such as Bitcoin and Ethereum require very many computres to mine in order to keep the network safe. Other blockchains such as Steemit uses 30 witnesses that need to "mine" the blockchain, and this witness-based blockchain uses a lot less electricity compared to the proof-of-work ones.
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Aaaaahhhhhhh. Hmm, that explains for me a bit about the witnesses that I keep hearing about!