Why would he say "but" in this context? What does it mean?

Native speaker’s answer
Rebecca
So, here, the phrase "but one" means "only one"! However, usually, this phrase means "except for" one or whatever amount. Here it's also been used for dramatic effect. Ex: There are but two activities we haven't done yet. = We've done all the activities except for two so far. = There are only two activities we haven't done yet. Ex: There is but one thing I'd like to say to you... => just one thing to say