There is an undo-module as standard in Calamus SL2003. But maybe you have not loaded it? This can be done under File/External Modules/ then you select Load and select the Undo-module in the file selector.
Note that not all modules in Calamus supports the undo-features of the undo-module. But the most important ones do support the undo.
The number of undo-steps you want is up to you. You define how many you want in the undo-setup. Memory being the only limit.