Form validation - callback function question

[eluser]Andrewkha[/eluser]
hi!

I have a question regarding form validation with the callback function.
The documentation says that I can pass the additional parameter like this

"callback_foo[bar]"

but what if bar is not static but variable?

For example, I'm developing a sports application.
Need to enter a game details, including the tour number, into the specific tournament. Each tournament has different number of tours which is stored in the DB.
So how can I pass the tournament id to the callback function to get the higher limit? Or how can I pass already retrieved from the DB value?