Lets call your app GUI.
First define a text box on the GUI;
TextEdit=uicontrol('Style','Edit','Units', 'normalized','string','YourNumber','Position',[0.02,0.84,0.05,0.02],'CallBack', @EditTextboxCallBack, 'HorizontalAlignment','center','visible','on','Parent', GUI);
%or 'Style','Text' instead of 'Style','Edit' if you dont want/need the user to change the number.
Then, lets say you have a matlab file named initdata, which contains just one integer. Load it, then set the edit box value to this integer.
myinteger=load('initdata.mat');
set(TextEdit, 'String', num2str(myinteger));
%If you need the user to be able to specify a different input, make a function with the same name as your CallBack in the uicontrol.
function EditTextboxCallBack(~,~)
myintegerstring = get(TextEdit,'String');
myinteger=str2num(myintegerstring);
end
