Can I limit the number of items put into a slot?
Off hand, I would say no. You could write your own slot content finder to only return the first item (or x number) put in it, effectively limiting the number of items displayed (you could keep putting item in the slot, they would just never show up anywhere)…I wouldn’t recommend this of course…
You can add a validation exit PSOValidateRelatedItems to the Content Type that contains the slot, adding values for the parameters so that a rule is placed on the slot for no more than 1 item. I think it is checked at workflow transiation time (at check-in possibly), and throws up one of those ugly, obtuse error messages if there are more than one item.
Slot is associated with or inserted to a template, NOT directly to the content type. I am curious if the idea you suggested would ever work …