99% of the time you should suggest an amount or a range. A friend in Wisconsin told me, “Scott, tell the missionaries to give me some idea of what they want me to do. I can’t read their minds!”
Without suggesting an amount or range donors can only guess at what you want him or her to pray about. And most likely, their commitment will be lower than you were hoping. But have a rationale for what you are asking, don’t just pick a number out of the air.