Satoshi's paper refers to UTXO pruning, but block pruning is not mentioned in that paper because it is administrative, and nodes store historical blocks only to serve other nodes. To enable block pruning, a number of megabytes "to allot for raw block & undo data" must be given, with any value being intended to be safe. The release notes for -prune explains how to reduce storage requirements by pruning old blocks, and warns that reverting this setting requires re-downloading the entire blockchain. There is no auto setting for N.