This example shows how to switch between 2 software serial ports.
You don't give us any idea how fast that you need to transfer data, but software serial is limited to 38400.
If you need that many serial ports, the Mega has 3 extra hardware ports.
If you need a lot of ports I2C is not the best way if there is any distance involved. RS485 may be a better alternative.
can be only 2 or 3 because these are the only interrupt capable pins
That is nonsense. Most all pins have pin change interrupt capability.