I've always heard it said that "polyglot" starts with the fourth language. The opinion is that to have a at-home (family) language, an outdoor (regional) language and a market (capital) language is too common to be considered truly exceptional. Once you pick up the fourth, you're getting into hobby territory.

Kató Lomb (maybe the first official "simultaneous translator" wrote about gaining, losing, and maintaining languages entertainingly and in detail.

edit:

"Polyglot: How I Learn Languages" https://www.tesl-ej.org/books/lomb-2nd-Ed.pdf

"With Languages in Mind: Musings of a Polyglot" https://www.tesl-ej.org/pdf/ej78/WLIM.pdf