The maximum position size defines how many contracts you can have open at any given time across all positions.
The only exceptions are the following micro contracts: MES, MNQ, MYM, M2K, MGC, which can be traded for up to ten contracts and it'll only count as one contract towards the maximum position size. For example if the max. positions size allows you 3 contracts then you can either trade up to 3 ES, 2 ES & 10 MES, 1 ES & 20 MES or 30 MES.
Please note that maximum position size is still subject to the progression ladder.