Someone doing webdev fullstack for more than 10-15 years will know at minimum ES + (PHP/Python/Ruby/Java/Go...) + SQL + HTML/CSS and a few utility languages.
So at least 4. If you combine webdev with something low level/embedded, you need at least one systems language, so you're at 5 languages you need to be proficient in.
Add one hobby language or a second web backend or systems language, and you're at 6 major languages.
7 is a lot. But 5 is plausible to be proficient in for someone who switches between webdev and lowlevel stuff to not burn out, or has a FOSS hobby.
I've shipped production code in 17 different programming languages. I wouldn't say I'm proficient in any one of them, they are all just tools to solve a problem and the knowledge of language specifics comes and goes. Need to hyper-optimize a DB query on an Oracle RAC cluster? PL/SQL. Need a shader? GLSL works fine. Need a webpage? HTML/CSS/JS. Need to build a 7' long flying robot fish? C. Programming languages are just tools.
In over 20 years of working in IT I never found a single webdev-type of person that can write an efficient SQL query by himself. Yes, most are able to write a query to bring the correct result, but I saw way too many cases where the perf test on a database with the expected production volume was running in completely inacceptable times and the developer had no idea how to fix that; for each version of SQL perf-tuning is very specific.
Also proficient != expert. I met enough developers that were brilliant in their work to be convinced that 5x developers are not a myth, but they are real, while rare, occurrences. For me a senior developer in X knows the ins and outs of that X to the level that his code is an order of magnitude better in term of efficiency, performance, productivity and security. A regular developer can be just proficient, but it is not what I wrote about.
So at least 4. If you combine webdev with something low level/embedded, you need at least one systems language, so you're at 5 languages you need to be proficient in.
Add one hobby language or a second web backend or systems language, and you're at 6 major languages.
7 is a lot. But 5 is plausible to be proficient in for someone who switches between webdev and lowlevel stuff to not burn out, or has a FOSS hobby.