I used to worry a lot about forgetting languages, but my attitude is different now. I am confident that even if I become rusty in a language, I can revive it with enough reading and listening.
I also think 'fluency' is very hard to define. You can be fluent in some areas but incapable of talking in others. For example, I can talk business in English, but I couldn't give a lecture on astrophysics, even if I understood the concepts, simply because the vocabulary wouldn't be there.
I don't know what's the biggest number of languages a human being can be fluent in but I think a reasonable number for a normal person who is neither a language savant nor someone who dedicates their whole life into learning languages is four to six. If someone is seriously into languages, the number can raise to ten or more but I'm talking about average people here. My husband is fluent in 4 languages and he's not even really interested in languages. I once had a colleague who was fluent in six languages and I was always in awe when I heard her talking on the phone and switching effortlessly between those languages. She had a Spanish mother, German father, was brought up in Italy, studied in England, and worked in China and France, so she spoke Spanish, German, Italian, English, Chinese and French. I don't know which language she would call her mother tongue. I guess Italian. And I'm sure she didn't have the exact same level in all languages but she seemed to be at least in C1 in all of them.
My definition of fluency is very strict. I say that I "speak" six languages (i.e. I have at least intermediate level): German (mother tongue), English, Japanese, Chinese, French and Spanish but I'm only fluent in two (German and English). Let's see if I can reach fluency in one of my other languages. I don't have big problems speaking thwm and I understand most of what I read and hear but my skills in the other languages are still lightyears behind my skills in English. When I reach the level I have in English in another language as well, I'll consider myself fluent. Been there with Japanese, but after 15 years of ignoring that language, my level dropped substantially.
I can absolutely go along with Ariel's comment about the two things: the way you define fluency and the fact that you can revive your language skills by putting a little effort in it.
Thank you all for your interesting and insightful comments! I agree it's often hard to tell whether you're fluent in a language or not yet.
And well, speaking about "astrophysics", to be honest, I wouldn't be able to give a lecture on it in my mother tongue either, hope that doesn't mean I'm not fluent in it:)
