[2020] prop problem

Are you getting the "You cannot place any more of this type" when trying to place a prop? If so, then you have indeed reach the limit. You can use a mod like Procedural Objects to get around that, as it lets you turn things into POs instead, which have their own limit.

Otherwise your only option would be to remove props you have placed to free up some. If you have areas that are decorated similarly, then you can make custom parks and place props in them. Props inside buildings do not count towards the limit.