1,000,000 (1999, Tamaki Saito)
800,000 - 1,400,000 (2001, Ogi Naoki)
1,000,000 (2001, Kiyoshi Kaneyoshi)
150,000 - 360,000 (2003, WMHJ 川上)
50,000 - 1,000,000 (2005, Dziesinski)
100,000 - 320,000 (2006, Jones)
60% of the population (2007, Hattori Yuichi)
These numbers were estimations mostly subject to ecology fallacy. Many had reported this based on an instinct out of clinical experience rather than true facts. The reason to this is due to (I) the inconsistent definitions given to hikikomori; (II) the experts' opinions/clinical experience but not community study.
With these biases, we need to be aware of (a) the figures and (b) the suggested causes and (c) interventions that currently practice. May the Lord lead us to His humble practice, shalom!