It has 201 direct dependencies, making it the 9th most directly-depended-on system in Quicklisp, one spot ahead of UIOP.

Methodology (yes, I could've stayed in Lisp, but I started by awking systems.txt...):

    (with-open-file (*standard-output* #p"deps.txt" :direction :output)
      (iter
        (for system in (ql:provided-systems t))
        (for name = (ql::name system))
        (format t "~a ~a~%" (length (ql:who-depends-on name)) name)))

    $ sort -h deps.txt | tail
    200 uiop
    201 iterate
    206 split-sequence
    274 closer-mop
    344 cl-glfw-opengl-core
    350 bordeaux-threads
    387 fiveam
    391 cffi
    408 cl-ppcre
    1006 alexandria