Hello Amit,
I don't think it is possible to change the character limit in captcha
If the user can't find a captcha because it's too messy, too long, then user can ask for another captcha, and that's how it works. Moreover, reducing the number of character would just reduce the effectiveness of captcha.
If you really want it to be 6 digits then you will have to create a custom captcha.
Thanks,
Manoj