You really need to see a couple contests to get a feel for what is expected.
I have a call for every vocalization that is asked for. Mostly mouth calls. I do use a slate for the tree call.
If you add anything to the call that is aked for, it better sound real and it better fit together. Just starting out I would stick to basics. When they ask for the plain yelp give them anywhere from 1-12 yelps, pause then do it again. Any more and it could be considered an assembly yelp. Sometimes judges will dock points for that.
Usually you will be able to do 2 series per call. Say they ask for a flydown. You can do two or you can do one only, most callers will do 2.
Keep it simple and go from there.