Antifa doesn't have membership...
... because antifa is a political movement not an organization. It's a bit like asking "how many members does BLM (Black Lives Matter) have?" It represents a fundamental misunderstanding of what antifa is.
Unlike white-supremacist organization the Ku Klux Klan or the National Rifle Association, antifa doesn't have a central organization or membership. It's a political movement dating back to the 1920 of regular people who choose to act out against fascism (hence the name anti-fascist, shorted to antifa), in whatever way they see fit to do so.
Some antifa activists do form small decentralized groups of like-minded individuals, but it's impossible to know how many such groups there are or how many "members" they have in total, and that wouldn't include all the individuals who are also anti-fascist.