The answer to the original question "how many bars can I wear," is "as many as you can load on without exceeding max gross on the aircraft." There are no laws governing the wearing of epaulettes, and there don't seem to be any general standards.
You might also consider the question, how many bars can you wear without someone thinking you're a twit. The consensus answer to that one seems to be zero.
The only flight instructors I have seen with bars on work for air-taxi/charter companies, so they're wearing the bars to be identifiable to passengers, not to students.
And anyhow, if you wear shiny bars, take 'em off your shoulders when you're off duty so that you don't accidentally put them in the washing machine with the shirt.