A cult can just be a few members
How many followers do you need before you're technically a "religion"?
There are really only two different religions, the true one and the false one. The false one is man made. The true one is given to us by our creator. Eph 4:5,6 "One Lord, one faith, one baptism: One God and Father of all, who is over all and through all and in all"
At the last census in the UK, a sizeable number of respondents put their religion as "Klingon". The UK government now recognise Klingon as a religion as a consequence (I confess that I always thought of Klingons as a extra-terrestrial race, rather than organised brainwashing). But all religions are created by man, so one adherent is enough, I guess. Until 1979 Muggletonianism still struggled on in the UK with one faithful follower. Read "Small Gods" by Pratchett to see how people create gods (and not the other way around).
I don't think religion needs followers.
Wouldn't you agree that for there to be followers, they would need a leader? Jesus plainly stated in Matthew 23:10 "Neither be called leaders, for your Leader is one, the Christ". Clearly, Jesus is the leader of true Christians. Therefore, his followers in a very real sense follow closely in the steps of their Leader, Christ Jesus, and not humans in a man-made religion. (1 Pet. 2:21)
If I remember rightly, in the UK, if 10,000 people put something as their religion, it has to be legally recognised.
(Although thinking about it that doesn't sound like a lot.)