Layla - since your b* are really just the x*, you probably only need one 2D array for x that will store the results for each iteration. Storing all the data in one array is preferrable to storing the data in multiple variables. Try the following
x = []; % alternatively you can pre-size this if you know the number of rows of each x*
[L U] = lu(A);
x(:,1) = b;
for k = 1:51
b = x(:, k);
d = L\b;
x(:, k+1)= U\d;
end
We iterate 51 times so that we calculate the x0,x1,x2,...,x50 i.e. 51 columns of x.
