The backtrace disagrees with you. It aborts due to an assertion that
the workspace name isn't NULL. No indication of "not enough memory"
or heavy memory usage when enabling 20 workspaces.
I ran into this issue when
increasing from 14 to 18 workspaces.
You could switch between spaces with the mouse but as soon as you
use ctl+alt+arrows metacity would crash with the assertion failure.
I found a way to get around this by naming the workspaces.
Currently I have 20 workspaces configured.
Alan