Private Blockchains

So I get the concept of consensus, so with a Bitcoin or Ethereum blockchain, you get the large-scale consensus via 1000s of nodes. It is the spread of these nodes that makes things secure, through distribution. 
But, how would you generate the same level of consensus (safety / assurance) in a private chain? If you had only a few nodes, how would this equate to decentralised security? Is there a minimum number of nodes needed?