How many finder action undos are possible? I'd like to understand how deep the queue one can go undoing their previous actions in finder?
Empirically, five, but not always. 

I wouldn't rely on it; if you disturb the queue in any way you can find it's just one.

Test by duplicating a small file 7 times, then undo til it stops... 5 steps.
Trash the dupes.

Test again, duplicate the file 5 times. After the 5th copy, 'break the chain' by then copying one of those files to another partition.
You're down to one undo step.