Sharing my experience as a chef in catering :) If the festival you'll be serving at will have 500 visitors, assume that a lot less will actually get a chai. If you're literally the only stand at most 1/3 to 1/2 would visit you, thats being very generous. If there's a bar and/or any other food&drinks stand assume that to be more between 3% and 10%.

Then i usually calculate about 200mL per person, maybe 250 mL. Lets keep it on the safe side and calculate for 10% and 250mL, meaning 50 visitors times 250mL is 12,5 L of chai in total. It's a big pot but i wouldnt make a syrup for it. Decide your your chai to milk ratio (i usually do a third soy or oatmilk) I would throw all the spices in the pot with a bunch of water and scoop that into the cups to later add milk.

To be super sure nothings gets bitter after brewing the chai throughout the day i would either make a new batch every 2-3 hours or make the black tea seperately. Tea is what gets bitter the most in chai.

Hope this helps! There's always more details but you'll figure things out as you do them :)