Reading a high-speed rotary encoder with a Raspberry Pi
2 years ago we designed the system with 14 quadrature encoders with 4096ppr running at up to 10RPM. We used FPGA to implement 14 bidir 24bit counters which were available to CPU via SPI.

It appears to me that second good way to control 8 motors is to use smart encoders (SSI type or similar, it depends on raspberry PI i/o availability).