The number that seems reasonable to me is a maximum of 5 – 7 times per day in four different categories.
1) One or two of your tweets can be self-promotional, meaning you’re driving traffic to a specific landing page on your web-site or talking about an upcoming event.
2) Provide tips that are not directly about you. For example, as a nonprofit, you clearly want people to give to you, which is where the self-promotional tweets come in. However, to be helpful without promoting yourself, provide general tips about charitable giving.
3) Include some re-tweets, which is promotion of other people and their work.
4) Try to answer questions. When I can find some extra time, I’ll do a search for questions I can answer from any twitter user, regardless of whether or not we’re following each other.
With these guidelines in mind, I suggest spacing out your tweets. No one wants to see a steady stream of just your account. Also, know that some of your readers may only view tweets occasionally. If it’s a different time of day than when you’re sending multiple tweets, they may miss everything you send which is another good reason to spread it out.