What is the maximum number of people that have been inside the TARDIS at once? Is it in the David Tennant series 4 episodes "The Stolen Earth" and "Journey's End", when all "The Children of Time" have gathered?
The TARDIS is capable of of being as large as she wants to be. Given the size of the spaces we have seen within it just within the episode Journey to the Center of the TARDIS, she is capable of maintaining any number of large spaces simultaneously without effort. Stacking an army within her halls or spaces seems completely reasonable.
As we have seen recently in Journey to the Center of the TARDIS, there are a multitude of various space held within. The Engine room of the TARDIS holds a collapsing star, an area which would have to be larger than an entire conventional or even Jovian sized world.
Apart from the ability to travel in space and time (and, on occasion, to other dimensions), the most remarkable characteristic of a TARDIS is that its interior is much larger than it appears from the outside. The explanation is that a TARDIS is "dimensionally transcendental", meaning that its exterior and interior exist in separate dimensions. In The Robots of Death (1977), the Fourth Doctor tried to explain this to his companion Leela, using the analogy of how a larger cube can appear to be able to fit inside a smaller one if the larger cube is farther away, yet immediately accessible at the same time (see Tesseract). According to the Doctor, transdimensional engineering was "a key Time Lord discovery".
There has been a wide array of information written on the interior of the TARDIS over the years and several fans have tracked exactly what spaces have been discussed in all forms of the show, both written and televised. A site called the Whoniverse discusses the named spaces of the TARDIS in great detail.
I can't speak for the Old Whoniverse, but to my recollection, the largest number of individuals present in the TARDIS would had to have been during the episode "Journey's End". At the end of this episode, most of the recurring characters throughout the new series help the Doctor pilot the TARDIS (he claims that the turbulence is due to the fact that TARDISes are designed to have a six-man crew).
During this part of the episode, the following people are present inside the TARDIS: The Doctor, Donna Noble, Martha Jones, Rose Tyler, Jack Harkness, Sarah Jane Smith, Mickey (Ricky) Smith, and Jackie Tyler. This sums up to eight individuals.
Since the existing answers seem to give numbers from 8 to 10, it's possible the latest episode "In the Forest of the Night" just set a new record.
Near the end of the episode--35:50 is when I paused--the Doctor is talking to Clara, Mr. Pink and a class of eight schoolchildren (if I counted correctly), all of whom are inside of the TARDIS, which adds up to eleven people.