Code:
$ cd /usr/ports
$ make search key="window manager" | grep Port | wc -l
49
$
That large count of window managers doesn't include the big integrated desktops like Gnome, KDE, or XFCE, nor the 3 built-in window managers.
There are many choices. I use i3, and I sometimes run fluxbox under Xephyr(1) inside an i3 workspace for stacking applications.