Hi Vicmizzy,
I really had to check the manual if I missed something as well. It is not elaborating on an option to have no number.
However, typically you get no number if you are on Frame zero AND the “Render-Settings”>Output is set to “Current” frame.
So far I can tell, any other combination of Manual, Preview or even All Frames, will introduce the numbers. The numbers it self can have different appearances, as in the “Save” part offered.
So, if you find it to happen outside of that “Current Frame Zero” definition, please let me know. (Of course Quicktime Movies set to one frame only have no numbering as well…)
All the best
Sassi