If one were to want to keep track of data from 80 - 90 accelerometers using arduino, what would be the best way to go about this challenge?
1) Use the I2C address select inputs as an "enable". If you run out of I/Os before running out of bus time, drive the enables from an I/O expandander. If your accelerometers are SPI, drive or use the expander to drive the chip selects.
2) Use a tree of slave MCUs (Arduino or otherwise) reading a number of accelerometers, extracting and batching the useful data, and reporting only that upstream to a master. Probably use whichever of I2C or SPI your accelerometers are not using (SPI can be much faster, so hopefully that).
Some accelerometer / gyro parts have an on-board processor which you may be able to program to do some of the data extraction for you.
You could also consider using an FPGA to collect the data. A commonly available one with enough I/O pins to have a dedicated connection to each accelerometer (and so read them all in parallel) would probably also have the internal resources to implement an Arduino-class soft core processor, if not better.