The Maximum Position Size is anywhere from 2-5 standard lots at any one time depending on the account that you have chosen. This is calculated across all pairs that you are trading. We do allow you to trade micro and mini lots. So if you were in the $300K Trading Combine you can trade up to 30 mini lots or 300 micro lots at a time.
An example: If you are trading 2 standard lots of EUR/USD and 1 standard lot of USD/JPY at the same time you would not be able to open up any more trades in the $300K Trading Combine as that would exceed the 3 standard lots allowed. If you do open another lot, your account would break a rule and be ineligible for funding.
MetaTrader 4 may not reject orders that exceed the maximum position size, so traders are solely responsible for ensuring that their account complies with this rule at all times. Please make sure to include any outstanding orders that may be hit when calculating your position size.
TopstepFX has created an EA that will allow you to better understand how many lots you have open at any one time. You can find that EA here.