So 7 master Arduinos and 1 slave Arduino.
This is a fundamental error in the architecture. It must be the other way around: one master and seven slaves. If that is not practical in your project, I2C is the wrong choice.
Wire.begin(8); // join i2c bus with address #8
Wire.begin(9); // join i2c bus with address #8
An I2C slave can have only one address, at least on the AVR hardware.