Depends on what strategy you are doing. And how big your account is and what your risk tolerance is to follow your system.
We teach 5% of account risk a day /6 per trade for Elite - this requires about a 3k account to follow it perfectly. Not required but easier. Not that you need that much cash at all to trade the Elite system just if you want to use proper risk management $3,000 x 5% /6 is about $25 a trade which is basically the risk on a one contract using a nadex spread or nadex binary trade using the elite method.
Often 5% is used on core
5-10% max on EPC - demo this down a lot before doing it
5-20% on news trades -demo it first so you know how to exit but these things rock