As you can read in the MPU-6050's datasheet, it uses the I2C address
X is the state of the pin AD0. If you wire AD0 to ground, the address will be
0b1101000, whereas if you wire it to Vcc it will be
0b1101001. That is equivalent to the addresses
That means, that - depending on how you wire AD0 - you can have 2 distinct MPU-6050's on the same I2C bus (you cannot have devices with the same address).
If you really need to connect more than 2 of them, you have to use an I2C multiplexer like PCA9544A, which will divide the bus down to sub-busses.
EDIT: Usually the address pins of I2C devices are set permanently by wiring them either to ground or Vcc. The sense of the address system is, that a slave device will stay at it's address, not changing it. But as Jot reported in the comments and as the Arduino Playground site of the MPU-6050 confirms, you can actually set the address pin of the MPU-6050 during operation.
You need 1 digital output pin on your Uno for each MPU, which then get's tied to the MPU's AD0 pin via a voltage divider. The last part is important: You need a voltage divider, because the MPU is a 3.3V device, while the Uno works with 5V. To avoid frying the address pin, you need to step down the voltage.
Then you would use the address pins to set all MPU's to one address (let's say
0x69). When you need to communicate with one of them, you toggle it's address pin, which will change it's address (in this case to
0x68). Then you can use the normal I2C workflow with this address (
0x68). When you are done, you can put this MPU back to the other address by again toggling the address pin. You can repeat this with any of the connected MPU's, which can be many (I think the limit here is the I2C bus itself with it's wires capacity).
This is a workaround and not really, what the I2C bus is meant for. But it seems to be valid solution. Just be sure, that the addresses, that you communicate with, are always unique. Contacting an address, which belongs to more than 1 device, will block your I2C bus.