It's a bit difficult to give exact quantities here as it will always depend on how many onions you use. I usually cook them down in one to two kilogram lots. Here's a few pointers:
* I usually add 1/2 - 1 tablespoon of oil for every 1-2 onions. You don't want to add too much so start small. You can always drizzle a little more in while they are cooking.
* Remember how many onions you started with! I divide my final mix into approximate amounts according to the amount of onions that I used. This way each portion will be approximately one onion and you can use as needed.