Its actually pretty simple to figure it out
Each chair is sbout 2 feet wide . 2 feet deep and you need another 2 feet in front of you for leg room meaning you need 8 Square Foot per person
Now multiply this by 10,000 chairs and you will get an idea how many square feet you need for the chair. Of course you actually need more space as you need aisles and room for the dias and the sound equipment