This is the 494th most frequent Chinese word.
号
"号" (hào): Number, day (date), or size.
In this sentence, '号' refers to the date of the month.
今天是几号？
What date is today?
Here, '号' is used to denote the number associated with an item, in this case a building.
我住在三号楼。
I live in Building No. 3.
In this sentence, '号' refers to a number being called, potentially for a queue or appointment.