The typical person who learns a constructed language is probably on their 4th language. I’ve no data to back this up, but here’s why I think so:
- Everyone speaks their mother tongue.
- Most everyone takes a foreign language as an elective or requirement in school. It wasn’t Esperanto or a conlang.
- People discover Esperanto while studying the language the studied at school and notice Esperanto is easy because they speak 2 of the language that Esperanto uses for source material.
- People who study Esperanto learn about conlangs and some of them pursue them further since they’ve already demonstrated above average ability to learn foreign languages.
So if you know toki pona or any other conlang, you probably speak or read three other languages reasonably well.
The percent of people who go down the list in the opposite is likely small. But I don’t have the data for it.