I'm not sure there's a limit; if so it's not stated anywhere that I can find. I typically see two setup for redundancy, and I assume it will always use the first one listed unless it's not available. The link below states, "There can be more than one helper address per interface."
http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipapp/command/iap-cr-book/iap-i1.html#wp1413119578
