It's 1/5 of your lootable total in the TH, once the amount in the collectors has been subracted.
Even if you have 180k in your storage and 10k in your TH, the first 50k total is all that can be raided. So 500 will be available in the TH, and 2000 in the storage. For some reason, the max used to be 2480 but now it's been bumped to 2500.
