There are no exact rules on this, but I do recommend not having more than 5, at most. We always need to keep an eye on the timing, so when there more than one or two speakers, it is best to give them a time frame within which to speak. It's also a good idea to have your speakers send me their speeches, so that we can avoid repetition and make sure each speaker is introduced in the right order, and with relation to what they are speaking about.