In my opinion, the number of languages a person can speak fluently without forgetting can vary depending on various factors, including individual aptitude, learning strategies, exposure, and practice. While there is no fixed limit, some individuals have demonstrated the ability to fluently speak multiple languages.
Hello, I think that it depends on to which linguistic family this or that language belongs to. If these are Germanic languages - from 2 to 5, for instance.
We can develop our fluency in the languages that have similar stems.