For FREE you can always use 1 Master and 1 Slave. If you need to connect more accounts, you can add them in combination with your FREE setup.
For instance, if you add 1 paid account, you will be able to use a total of 3 accounts:
The 3 accounts above can be linked together in the trade copier
|1 paid account (Slave or Master)|
|1 free Master|
|1 free Slave with limitations|