Danny, Justin is spot on - there is no maximum.
The largest board I have seen 20 columns and it was a real pain to read on even a 24" display. I have also seen 14 columns spread across 4 42" LCD TV's for a really large JIRA Wallboard and that worked really well.
I don't believe that there's a system enforced maximum, but depending on your screen real estate, I'd put somewhere around 10 as the max (and even that makes issues very small). We had a board with 9 columns that we ended up splitting into 2 boards, so that it was easier to read.