I know amazing polyglot programmers who just know 3-4 languages.
I believe mastering two languages with complementary strengths would be far preferable to just syntactically learning a bunch of languages.
Stated another way, I believe if someone can program at an expert level in one lisp, they probably don't need to insist on learning more of them.
If you progress to "merely adequate" in a smorgasbord of languages but all you can do is churn out CRUD apps but couldn't implement basic algorithms for a search engine you've plateaued.
As a related note, learning new syntax is by far the least useful aspect of learning a new language. Coincidentally, it's also the easiest element to pick up and takes little time at all. Incorporating new paradigms into your way of thinking takes far longer.
I've also designed and implemented a few procedural, object-oriented, data-flow declarative, and logic meta-programming languages over the years. It is quite easy to get carried away with any of these paradigms, which is why I try to ground myself to working mostly with procedural imperative languages with aspects of other paradigms sprinkled in.