This isn't quite accurate. The 9-12 number was given by survivor Samuel Rajzman to the U.S. house committee on foreign affairs in 1945. Later research would find closer to 100 survivors. Also the number of deaths at Treblinka is disputed and may be as low as 750,000 or as high as 900,000. Focusing on exact numbers though is unnecessary. The story of Treblinka is the same whether the numbers are 750,000 and 12 or 900,000 and 100.

The survivors were all sonderkommandos (prisoners forced to work in the death camps). The sonderkommandos noticed a decrease in the number of trains coming to Treblinka and rightly assessed that the camp was going to be liquidated. They organized a resistance and on August 2, 1943 launched a revolt. There were about 700 sonderkommandos in the camp. About 100 directly engaged in a firefight using weapons stolen from a weapons storeroom. The other 600 attempted to escape (not cowardice this was the actual plan) Somewhere between 200 and 300 managed to scale the barbed wire and escape into the countryside.

The Germans launched a massive manhunt. This manhunt captured all but about 100. All persons captured were immediately executed. By the end of the war 70 survivors were confirmed to still be living. Most of these survivors joined polish resistance groups.

One of the first accounts to be published about the war was Grossman's 'Hell at Treblinka'. https://dl.dropboxusercontent.com/u/47875651/TheHellOfTreblinka.html

The article should be taken with a large grain of salt, as Grossman was writing when the facts were only just being unearthed and in an environment thick with Soviet propaganda. For example, Grossman didn't quite understand the way Nazis exterminated and misunderstood testimony. For example, descriptions of 'steam' being used was actually a misunderstanding of Ziklon B (which created a steam like mist). He also mistook medical evidence about oxygen deprivation, thinking it meant that the Germans pumped the oxygen out of the room. This was instead caused by the Cyanide and the pumps were to remove the gas from the chambers so that the Kapos could clear them. Those mistakes do not make the rest of his article invalid, but it shows how little was known by the end of the war.

His account is still worth a read, as it is brilliantly written and deals with the immediate understanding of the horrendous crime. One of the survivors mentioned is that of a carpenter, who hid in a ditch when he could. He survived and told his story. Most other survivors come from a famous escape attempt, using captured weapons. This is detailed quite well by /u/iringin. Otherwise, the second main source comes from lucky individuals, who successfully took an opportunity to escape or hide.

The estimates for Treblinka are quite hard to piece together. The camp was successfully deconstructed (with records destroyed) and it is difficult to understand which people went where within the KZ system- especially within the extermination camps.

I'm amazed at how thoroughly (and almost completely) the Nazis liquidated Treblinka and all of its associated evidence.

Is there any possibility that there were death camps that we still don't know about, because the Nazis completely erased all evidence of its existence?