genetics - How can Southern blotting be used to establish gene copy number?

I am reading a paper on a certain marine microorganism that can eat alkanes and therefore might be useful for cleaning up oil spills, etc. This sentence struck me as odd (although I am sure that the answer is pretty simple):

Using the same probe, Southern blotting analysis showed that there was only one copy of alkB1 in the B-5 chromosome.

I realized that I don't know how Southern blotting can tell you about copy number. Can someone help me on this?