2 L where, L is the number of loci. Ok so now that we have the gametes we have the individuals. So if you are given the results of the testcross then you can figure out the F1 gametes by reversing this process. So the next step is to identify the parental types. Calculate recombination between loci.
Recombination is a pairwise measure of genetic distance.
So we can calculate it between every pair of loci. Order the loci. A and F are the farthest apart. Let's do a group exercise. Make the F1, do a testcross come up with all possible offspring of the testcross.