Here is a press release about what is claimed to be the first comprehensive sumatran tiger survey. The article says there is some good news because tigers occupy 70% of available forests. But frustratingly, it does not give an estimate of the actual number of tigers. Nine NGO's sponsored the study, including three zoo based organizations (Bronx/WCS, London/ZSL, Durrel/Jersey).
http://www.fauna-flora.org/wp-content/uploads/New-hope-for-Sumatran-tigers.pdf
http://www.fauna-flora.org/wp-content/uploads/New-hope-for-Sumatran-tigers.pdf