Yes, it is not changing the file name.
it's 455 files, but in groups of 34, so fare.
I know, of course not.
That is why I asked you to check whether the DISCNUMBER field contains the correct number for each group that should have the same number.
I think you still have to define the "Next disc on change of" where I would suggest TITLE.
Do not tick "Save count of discs".
To check if the result is OK, see the extended tags dialogue Alt-T - and be so kind to share a screenshot of that dialogue for a single file.
Thanks for the feedback, but I thought there was an easy way to do it, that someone had already tried on the filename itself 
The problem is not the manipulation of the filename - but the special grouping that you have in mind.
No program can guess where you want to increase the number unless you give a criterion that tells the program when to change something.
It is a pity really, that you did not supply the requested information.
So this means in the end that you either have to do it manually, half automatic with the $replace() function or with a different program for file handling.
Yes, I think you are right, but there is no increase in number, every croup of name will have same number.
Here is another attempt:
If the current numbers are correct, only that they have to be increased by 1, then try
Convert>Tag-Tag for _FILENAME
Format string: $num($add($num(%_filename%,3),1),3) - $regexp(%_filename%,^\d+ - ,)
No different outcome, I do not think that program work on file names.
It worked, I just had to type the name at the top as I couldn't choose from the list, thank you very much
Can you explain to me how the 3 numbers work? I entered 1 and 2 and it worked fine, but from 3 on it was wrong.
Where did you enter numbers?
My format string was intended to add a 1 to any leading number and write that back. So 5 > 6, 4 > 5, 3 > 4, 2 > 3, 1 > 2, 0 > 1.
In the middle, I tried a little again, and I got it to work.
The hyphen in the original filename is no "minus" character but something else (looks longer).
So the original number does not get deleted as that checks for a pattern "number followed by space and minus".