I would plan for 400, expecting 300 providing the event is at night. I would also forget about the ribs unless you have a strong reputation; they're too hard to eat unless there is somewhere to sit down.
Questions that may impact planning:
Is that 20k in one day or over several?
Afternoon, evening or all day event?
How old is the event?
Do people have to pay to get in?
How come your friend isn't still doing the event?
What is your reputation???
A couple things to consider:
If your barbecue is good, your operation is good and you have a reputation, you will outsell most other food vendors.
You need to have a plan for properly handling extra product so that it does not become a loss or a danger.
The "Daddy" in Smoke Daddy's Barbecue