Austin Fire Department spokesman Chayer Smith said he expected the figure to change "substantially upward" once firefighters can survey the area in the daylight.
Authorities believe a homeless man started the fire by lighting a campfire to cook amid strong winds and tinder-dry conditions that have left firefighters battling a spate of wildfires threatening communities across Texas.
Wildfires have spread across more than 700,000 acres - about the size of Rhode Island - in drought-stricken Texas. Those conditions endured Monday.
"It's so dry statewide, it's just amazing," Texas Forest Service spokesman Marq Webb told The Associated Press.
New blazes have been detected Tyler and Hardin counties in East Texas, where about 3,000 acres have burned in an area known for its thick forests. Webb said he was particularly concerned that the tops of some trees have been set alight by wind-carried embers, a type of blaze known as crowning.
"Fire builds up and gets into the tops of trees, it just races through the top," said Webb. "You don't normally see that kind of fire activity in East Texas. The fire factor is just running off the charts."
The fire in southwest Austin destroyed at least eight homes and damaged 10 others. The suspect, whose name has not been released, is charged with reckless endangerment. He is being held on $50,000 bond.
About 200 homes had been evacuated, but many of those residents were being allowed back in late Sunday. Two streets were still closed to traffic and residents, Smith said.
Lindsey Senn fled her house after hearing neighbors banging on doors and shouting warnings. The 22-year-old said she looked out her door, saw smoke everywhere, grabbed her pets and left.
"Am I going to come home to a house? ... A lot of thoughts are going through my head right now," Senn said.
Although Smith said the Austin fire was contained, Texas forestry officials say the potential for wildfires was expected to remain explosive in West and northwest Texas on Monday.
The wildfires have ravaged more than 1,000 square miles of mostly rural terrain in the last week, prompting Gov. Rick Perry to ask President Barack Obama for federal help.
"Texas is reaching its capacity to respond to these emergencies and is in need of federal assistance," Perry said in a statement Sunday.
Meanwhile, the Texas Department of Public Safety said a volunteer firefighter who died Friday had been hit by a vehicle after fleeing a fire truck trapped in a wildfire-consumed pasture between Fort Worth and Abilene.
A preliminary autopsy report from the Tarrant County medical examiner in Fort Worth said Eastland volunteer firefighter Greg Simmons died of blunt force trauma, Senior DPS Trooper Phillip "Sparky" Dean told The Associated Press on Sunday.
Eastland officials initially said the 50-year-old firefighter died after being overcome by smoke and falling into a ditch.
Several wildfires 70 to 80 miles west of Fort Worth around and south of Possum Kingdom Reservoir had burned about 32,000 acres and may have destroyed more than 50 homes as of Sunday evening, said Forest Service spokeswoman Victoria Koenig.
In the Austin fire, two C-130 aircraft made several flyovers, dumping fire retardant over the fire. In the evacuated neighborhood, many residents had left sprinklers running in hopes of avoiding fire damage.
Inside the Southwest Hills Community Church, being used as an evacuation shelter, residents waited and American Red Cross volunteers brought water and other provisions.
"They are grateful to have a place to go," said church pastor Greg Hill. "They're wondering the state of their homes. The worst part is waiting. Information has been slow. People are being taken care of. They have all the comforts of home without being home, but their minds are on their homes."
Associated Press writers Terry Wallace and Diana Heidgerd in Dallas contributed to this report.