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, MCL, M6E, MHG 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.