Super Saiyan 11 is another form of Super Saiyan. It can easily be achieved if you have accomplished 8 other Super Saiyan forms. If you have already trained in those forms for about one month, you will be able to turn into Super Saiyan 9 and 10. Then you have to train in the Super Saiyan 10 form for a day and on the next day, you must raise your Ki as high as it can go in Super Saiyan 10. If you do all of this correctly, you will be able to turn into the Super Saiyan 11 form.
You can unlock this form in the video game, Dragon Ball: Raging Blast 5. You have to unlock every other Super Saiyan form in this game so just every other form before this since it's the last form of Super Saiyan in the game. If you unlock this form you will be able to use it with every Saiyan in the game.
If you have Raging Blast 5 save data on your Wii Console, every Super Saiyan form will already be avalible in the shop so you will have to get a certain amount of ryo to buy it. If you don't have Raging Blast 5, you'll have to unlock every other SS form and once you have puchased them from the shop the form will be avalible and then you can buy it
The same thing with Super Blast 1.