I did do filter coffee for 30 out of the back of the Berlingo one year on the 400
The usual stop in Minehead wasn't available that year
I saw them off at the start then went back home by car, made several litres of coffee, put it in large vacuum flasks then drove up to Minehead, somehow getting there minutes before Toby. Baggy had made a load of rather nice biscuits to go with it.
If I had more vacuum flasks ( I bought 2 or 3 large ones from Wilco ) I could have done 100 people easily. After 50km, even though it was an easy 50km, the pack had split up nicely. As they came in in 2s and 3s it wasn't difficult to keep them all supplied with coffee and the cards stamped