How can I change the number "3" over the triplet symbol when I use this tool with 6 notes instead of 3?
An interim working solutions from the standpoint of printing a score would be to simply not use the triplet tool as such and rather use a slur with the notes in question with a "6" added over the slur through use of the "text" function under the insert menu.
This would obviously not work for "playback" purposes. It would also not work even for score printing purposes if there are multiple staffs at that point, some of which do not contain the sextuplets, since measure bars would then undoubtedly be off.
I agree with the comment that a program alteration could be useful with some added flexibility. The foregoing, however, might occasionally allow an interim working fix for score printing purposes.
Ivan's questions suggests a possible NWC enhancement: Why not have the "triplet tool" generate a number equal to the number of equal-duration notes that the user selected? In the case of notes of unequal duration, use the shortest duration and compute the number of notes of that duration that would fill out the time. e.g. selecting over one eighth note and three 16th notes would produce a "5" for the implied five 16th notes over 4. (Is this clear?)
The suggestion of a text-inserted number with a slur in the score can even work for playback under some circumstances. If there are no other staves or voices with notes in the normal rhythm, then you can get playback to "work" by inserting hidden tempo changes before and after the Xtuplet notes.
When I have to beam more than 3 notes (triplets), I highlight the amount of notes to be beamed and press control B. This gives you the correct number of notes with a beam, but you would have to write in the number, or use text insert.