As at Tuesday this week the number was 9325.
Mr Lightfoot said with the prison population increasing, Corrections was looking at options for "maximising capacity" at prison sites.
He said prisoner numbers were dependent on factors outside the department's control, including legislative changes, judicial decisions, policing trends and crime levels. He said the current spike in inmate numbers was due to more people being held in prison on remand than was forecast.
"Legislative changes have also meant prisoners serve more of their sentence in prison, and there has been an increase in people serving longer sentences for more serious crimes."
Mr Lightfoot said the department had been managing to work within existing prison capacity by making the most of existing cell numbers, deferring planned maintenance work, and reconfiguring prison units.
But he said these initiatives were not sufficient to manage future increases.
The latest forecast predicts that increasing volumes will continue for the foreseeable future, rising as high as 10,000 by 2017.
"Given the long lead-in times required to add new capacity, we need to look at options and plan ahead now," Mr Lightfoot said.