Gauging the size of the community is difficult, especially since there's no strict definition of what being in the community means. The number of people that own costume tails is one indicator, although there's no strict definition since some people want tails but don't have them and some people have tails but don't care about merpeople, like people who might make an Ariel costume as part of cosplaying Disney princesses.
At any rate the community can be estimated to be in the hundreds or possibly thousands based on what definition you use. This is estimated based on online activity of the community, interest in the World Mermaid Awards, and estimated sales volume of professional tailmakers.