As a beginner I'd like to ask if I want to connect multiple flex sensors to Bela, whether I need 9V battery or microcontroller voltage will suffice for optimal readings.

You can power the Trill sensors from 5V max, but when used with Bela you should use 3.3V. You can use as many sensor as they fit on
each I2C bus (typically 9 before you run out of addresses).