The good (?) thing is that you can absolutely forget languages -- I used to, as a small child, speak both Cantonese and Hokkien fluently; after more than a decade and half of not speaking or hearing a single word, I have now completely lost the ability to do so. I was in Hong Kong right before COVID and was flabbergasted that I literally couldn't understand most of what I was hearing and was completely stuck when trying to speak Cantonese.
I'm not sure if speaking more languages exhausts some kind of mental capacity -- that's not my experience at all. There was a time when I spoke nothing but English for years, a time when I spoke 4 languages, and now I speak both English and Chinese daily; I haven't really observed any differences in my proficiency. My Chinese proficiency has probably gone back up to native status after atrophying in my college years of not speaking a single word.
I'm not sure if speaking more languages exhausts some kind of mental capacity -- that's not my experience at all. There was a time when I spoke nothing but English for years, a time when I spoke 4 languages, and now I speak both English and Chinese daily; I haven't really observed any differences in my proficiency. My Chinese proficiency has probably gone back up to native status after atrophying in my college years of not speaking a single word.