We are currently in the 115th U.S. Congress that started in January of 2017.
How is it that the Tax Cuts and Jobs Act is assigned #1 in its bill name (H.R. 1) when it wasn't introduced until 9 months after this congress began?
From Wikipedia (emphasis mine)
Every two years, at the start of odd-numbered years, the United States Congress recommences numbering from 1, though for bills the House has an order reserving the first 20 bill numbers and the Senate has similar measures for the first 10 bills.
So this bill was given a special number for the purposes of recording.
Per the Congressional website
In recent Congresses, the resolution specifying House internal rules of procedure includes reserving bill numbers for assignment by the Speaker. In the 112th Congress (2011-2012) the practice was extended to reserve additional bill numbers for assignment by the Minority Leader.