You are welcome, @nkabir.
I used the tutorial from @gordon and edited sources obtained from ubuntu repository:
Yes. I had to edit 3 files:
marco/src/core/prefs.c--> redefineMAX_REASONABLE_WORKSPACES;marco/src/org.mate.marco.gschema.xml--> changenum-workspacesrange AND editorg.mate.Marco.workspace-namesschema, add keys for all extra workspaces by hand ;mate-panel/applets/wncklet/workspace-switcher.ui--> editnum_workspaces_adjustmentobject .
Then I compiled marco and mate-panel and installed all created .debs.
This was unnecessary. Looks like I had the schemas recompiled automatically during .deb installation.