I wouldn't worry too much about forgetting parts of languages you already know. You might forget a language that you don't use, but I've never forgotten a language because I learned another. If anything, the opposite is true: learning another language gives you new insight into one you already know.
To address the question "How many languages should I learn?", you should consider what your goals are. If you want to get into web development, you need JavaScript. If you want to get into iOS, you need Swift or Objective C. If you want to get into data science or scientific programming, you should probably learn Python. If you are just looking for any programming position (especially entry level positions), your employer will probably care more about your software engineering knowledge than which languages you know. I got my first internship writing JavaScript without having any prior JavaScript experience. As a final note, if your goal is to become a better programmer, I'd recommend learning C, a Lisp (my favorite is Clojure), or a static functional language (Haskell, OCaml/ReasonML, Elm). You may never use those languages at your work, but they change the way you think about programming and make you a better programmer.
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I wouldn't worry too much about forgetting parts of languages you already know. You might forget a language that you don't use, but I've never forgotten a language because I learned another. If anything, the opposite is true: learning another language gives you new insight into one you already know.
To address the question "How many languages should I learn?", you should consider what your goals are. If you want to get into web development, you need JavaScript. If you want to get into iOS, you need Swift or Objective C. If you want to get into data science or scientific programming, you should probably learn Python. If you are just looking for any programming position (especially entry level positions), your employer will probably care more about your software engineering knowledge than which languages you know. I got my first internship writing JavaScript without having any prior JavaScript experience. As a final note, if your goal is to become a better programmer, I'd recommend learning C, a Lisp (my favorite is Clojure), or a static functional language (Haskell, OCaml/ReasonML, Elm). You may never use those languages at your work, but they change the way you think about programming and make you a better programmer.