presumably you mean "tonal range".
Let's do a thought experiment. Let's take a gray card outside on a "sunny 16" kind of day and take a reading of the card. Next let's take 15 shots of the card and label them -7 through +7. Frame -7 is to be underexposed by 7 stops, -6 by 6 stops and so on, with frame 0 correctly exposed and frame +7 overexposed by 7 stops. Now lets make 15 8x10 prints (the size of our gray card), likewise labeled prints -7 through +7. Since this is a thought experiment let's grant ourselves the luxury of assuming that our print paper has infinite tonal range and any limitations come from our film. Let's also assume that our lab does not make any mistakes in temperature, timing, etc. They're perfect.
Theory tells us that print 0 should match the gray card. Print -7 should be black with absolutely no texture. Likewise print +7 should be white with no texture. Furthermore prints -7, -6 and -5 should all be black and indistinguishable from each other whether by visual inspection or by densitometer measurement. Likewise prints +5, +6 and +7 should be white and indistinguishable. If this is the case then print -5 is Zone 0 (maximum black) and print +5 is Zone X (maximum white). Prints -6 and -7 are blacker than black and print +6 and +7 are whiter than white.
If all the above is true then I would suggest that the "tonal range" of our film is 11, i.e., it displays 11 distinguishable tones, one stop apart, between and including maximum black and maximum white. Anything darker or brighter cannot be distinguished from Zone 0 or Zone X respectively.
However, suppose that we cannot distinguish print -5 from print -4, and that we also cannot distinguish print +4 from print +5. That would mean that -4 is maximum black, +4 is maximum white and our tonal range is 9. How can this be? Many explanations come to mind: 1)The zone system is totally invalid. 2)This is crappy film; don't buy it again. 3)The film speed
on the box is not the speed of the film
in the box; 4)The camera or the meter or both are off and the actual spacing of the zones is not one stop; etc, etc.
At this point my train of thought has reached the switch yard and doesn't know where to go next.