SliderStep = 0.04 gives you 26 steps because the change in the slider's value from one click is
(Max-Min)*SliderStep = (12-(-12))*0.04 = 0.96
So it takes 24/0.96=25 clicks to go from min value to max value. (Note that there is one more step by your counting because you are counting the number of states the slider is in as you click from min value to max value, and I am counting the number of clicks in between min and max.)
Note that 25=1/0.04, i.e., the number of clicks from min to max is the inverse of the SliderStep. Therefore, in order to have each click change the slider value by 1, you want it to take 24 clicks to go from min to max, which means the sliderStep should be 1/24.
set(handles.slider1,'min',-12);
set(handles.slider1,'max',12);
set(handles.slider1,'SliderStep',[1/24,1/24]);
Or more succinctly:
set(handles.slider1,'Min',-12,'Max',12,'SliderStep',[1/24,1/24]);
(By the way, I don't think it makes much sense to set those properties in the slider callback. Typically you'd set them in GUIDE or in the OpeningFcn, or, if they depend on other stuff, set them in the callback of the stuff that they depend on or a related function.)
For the other question, how to display the slider's value in a static text, first create the static text (e.g., in GUIDE), say it's called handles.text, and then in your slider callback, set the text's String from the slider's Value:
function slider1_Callback(hObject, eventdata, handles)
set(handles.text,'String',num2str(get(handles.slider1,'Value')));
end
