They are exactly the same in meaning. "But" used in this way is called a "limiting modifier". "But one" is just a little more poetic than "only one". This means it is used more in writing than in speech. There are but three more apples remaining. There are only three more apples remaining. That is but one memory of many that I cherish. That is only one memory of many that I cherish. Getting help is but a phone call away. Getting help is but one phone call away. Getting help is only one phone call away.