If the Tournament is still in draft mode, you can make any changes you like.
If the Tournament has gone live and has registered developers that have selected their coding language, you’ll no longer be able to change languages or the number of Challenges. That’s like moving the goalposts which is a bit of a no-no.
You can, however, still make changes to things like name and description.
Note: For more about tournament management, check out this article: How to Create a Tournament