If you're writing anything REAL for the web, you typically are; client side (JavaScript) and server-side (PHP, ASP, Python, Ruby, pick one)... So I think that number would be higher than you think.
Me, I actively use PHP, JavaScript (both client and server-side), Pascal, C++, and x86 Assembler (using NASM) -- admittedly for different tasks; one set of tools for web development, another set for working on Arudino/Teensy projects, yet another for working on my various retrocomputing hobbies. Since apparently I didn't get the memo nobody writes games for DOS anymore...
I have to agree with @Edouard that switching between languages can be beneficial -- more so the "lower level" you go. You start playing around at the machine language level it gives you an insight into how things REALLY work. That insight (particularly if you learn how to build a compiler or interpreter) can reap real benefits particularly when certain groups of developers start talking out their backsides about things like pass by reference being "bad" in interpreted languages like PHP... You quickly start to recognize the outright BS a lot of "experts" spew because they've never worked in anything more complex than jQuery and PHP copypasta.