First you should store the data-sets in some kind of array. For the most tolerant storage (that doesn't care about different dimenstions or data-type) use cell-arrays. Then you can do somethinh like this:
figure
for n = 1:12
curr_I = I_all_in_cell{n};
subplot(3,4,n)
histogram(curr_I)
hold on
end
Do not use numbered variables for this type of work, use the different arrays that matlab makes it possible to work with. Just build the array instantly at the point where you load the data. Also look after the numerber of times this type of question have been asked for a more in-depth explanations.
HTH
