How long of a wire and how big of a load can I have for SPI bus?
To answer your main question: No, SPI TTL or CMOS signals can not reliably drive 128 loads via off-board cables.
There is no easy calculation you can do to show this. It has to do with both loading, reflections and ground references. You can simulate the propertis of such a system, but it is a complicated waste of time.
What might work reasonably well is to use RS422 drivers and receivers specified for as many loads as you need. You also need to terminate each differential pair at the furthest end.
If you have multiple SPI slaves per board, then they can share RS422 receivers.
You also probably want to at least partially daisy chain your SPI slaves. For example, all slaves on a single board could form a chain. It will greatly reduce your cabling.
This way you will have differential signaling in your cables, which can handle more loads and is a way more reliable signaling scheme in cables.
It does however double you number of wires..
But a disclaimer is in order: No one in their right mind would design a system like this with 128+3 signaling lanes.