The Guinness Book of Records says that the official record is 152 years, although it has said the same thing for decades. The 152-year-old was accidentally killed in 1918, and was in fact probably much older than 152 because it was already fully-grown by the time its history can be traced.
The problem with this question is that it is very difficult to be able to prove a continuous line of ownership/ custody/ whereabouts and identity of any specific individual over the extended period which would be required. As rinkytink said, it's quite likely that there have been many living up to 200 or more.
On a tengential point, I may be wrong but I believe that reptiles like tortoises and crocodiles etc. don't actually grow old; they just carry on growing bigger and bigger until they die from illnesses or accidents or predators etc. So there may be no limit in theory to the possible age of a tortoise, as long as there is enough food and safe living conditions.