As a child, you normally begin with just one instrument.
Still, almost any professional classical orchestra musician (and most skilled musicians in general) learned basic elements of another instrument at some point in their career, possibly a few years after beginning to play their instrument - assuming they didn't change their preference.
This is not a rule, though.
String, percussion and wind players probably studied piano for a short period of time (statistically, between 2 and 5 years), because that's the convention for professional music courses in those families. But this normally happens at least 3-4 years after beginning their path as musicians.
The reason behind it is that those instrument families lack some important aspects of music learning: most importantly, better knowledge, understanding and self-awareness about polyphony. Knowing more about these aspects normally improves their capabilities in their own instrument(s).
Piano players sometimes learn to play another instrument, but it's not the norm: it is mostly up to their choice, the suggestions of their teachers, or the possible requirement of their courses.
This is often the case for harp, organ and guitar too.
Be aware, though: learning another instrument is not a guarantee of becoming a better musician. I know some musicians that have studied other instruments, and are no better than other fellow musicians that only studied their own instrument, or just another "extra" one.
I've also met a few that even took degrees for many other instruments, but that clearly didn't make them better in any way.
Learning to play another instrument, especially in young age, is certainly an important added aspect to the knowledge and experience of what could become an highly skilled musician.
But it doesn't automatically make them better musicians, and not even make them in the "top 500" (whatever that may mean).