Free plan has limitations since September of 2021.
With free subscription you can only have 10 (local/stored) session and you can only have 5 parallel sessions open at the same time. You can have one synchronized session.
There are no limitations for temporary sessions.
If you would like to have unlimited local and stored sessions, unlimited parallel sessions, you have to upgrate to at least Premium plan.
You can upgrade here: https://sessionbox.io/plans/auto-renewal