I agree with Cheri - I tweet for @thewomensmuseum and I do try to limit my wide-broadcast tweets (where I share program information, museum facts, links from other users, etc.), but I think that this point should be clarified - 5 to 6 tweets a day *not including* replies and other conversational messages.
That point aside, great article! It's a great resource for non-profits just starting out on Twitter. I especially love the advice to follow all followers back. Using a program like Tweetdeck allows me to make groups from our followers while still show that I am open to conversation with everyone that wants to work with us.