 You may think the number of people at any given time (or at least the number of human people) is simply the number of human organisms there are then (ignoring any that don’t count as people). But this is disputed. Some say that cutting the main connections between the cerebral hemispheres results in radical disunity of consciousness, and that because of this, two people share a single organism (see e.g. Nagel 1971; Puccetti 1973 argues that there are two people within each normal human being; see also van Inwagen 1990: 188–212). Others say that a human being with multiple personality could literally be the home of two or more thinking beings (Wilkes 1988: 127f., Rovane 1998: 169ff.; see also Olson 2003b, Snowdon 2014: ch. 7). Still others argue that two people can share an organism in cases of conjoined twinning (Campbell and McMahan 2010; see also Olson 2014).