I haven't a clue where this was copied from but I feel that it was from this site. Mmmm, there's a problem. In the book I have made it more clear than on the website but the recipes DO NOT scale up well beyond being doubled. That is because they are not scaled down versions of restaurant procedures but rather a method I devised to make domestic sized portions.
If you are making curries AND vegetable dishes then you WILL need the equivalent of 10 times the amount of basic curry sauce (BCS) to make a meal for 20.
Scaling up the curry recipes that much will NOT work either. What will happen is that you will get a kind of curried stew rather than the stir-fried effect of restaurant style curries. Also the spicing will be all wrong. You would certainly need to reduce the stronger spices, especially chilli powder, a fair amount. Getting the curries too wet will be a big problem and water should be added in very small amounts.
So, what to do?
You could hunt out some more traditional recipes where the curries are cooked for a long period of time and in larger quantities but that will not produce restaurant style curries which is what I expect you are after. Even a restaurant would probably make 10 x double portions of curry if you turned up and ordered the same curry for 20 (not the BCS, of course, that would have already been made in bulk earlier on in the day).
If you are committed to making restaurant-style curries then I can only suggest what I would do in the circumstances although, as you rightly point out, I avoid doing it like the plague and have never stretched to 20 (12 people is about my limit)!