Number of towers definitely increases the speed that your culture spreads and pushes enemy culture back more easily so if you're dealing with an annoying turtler or you'd prefer they come to your turtle, buildling 8 culture structures on a fully developed neighboring arid world should get their attention rather quickly.
I'm not sure multiple towers will actually enhance the distance unchallenged culture travels though. Population might have something to do with that. I'm pretty sure one tower will always spread culture one phase lane away when unchallenged though.
One thing I am certain is that multiple cultures travelling through a choke point will kick the crap out of whatever is on the other end. Try an AI game of centrifuge with the Advent. Take the inner worlds only and build a ton of culture buildings and lots of defenses. It's over pretty quick since everybody's homeworld is only two phase lanes away from every culture structure you build. Even the AI seems to get pissed off at the strategy.
My general approach is to build one tower on every non asteroid world and multiple towers where needed. Culture isn't necessarily a great offensive weapon but it can be a great way to force your opponents to attack you on more favorable ground.