The published numbers of Bible Student/JW members over the decades since the '20s are not likely to be of much use, as the definition of 'member' has continued to change. JWs have always struggled with defining 'member'. This was one of the things that Rutherford and his seven accomplices got into trouble for during WW1 and found themselves in prison. The Bible Students previously had always said they weren't a religion, didn't have a clergy, etc. and never bothered with defining membership. Then they issued affidavits for men called up under the selective service asserting they were members of a pacifist religion. After WW1 they changed their stance and required people to provide a record of preaching work, which I believe was motivated at the time to create a written record to substantiate 'membership'. It was all about the draft.
In recent decades (post WW2), membership has been defined as 'regular publisher', not attendance at meetings, not baptism, but publisher with a written (now electronic) service record.