SPI is a chain of flip-flops.
There is no hard limit to the length of the chain. There is no addressing. It's not like I2C where you have a limited number of addresses.
The soft limit is how long you're willing to wait to clock data through the whole chain.
If you have a chain of for instance 1000 8 bit devices, and are serial clocking at (for ease of calculation) 8MHz, then you'll have to wait 1mS to update the entire chain. That would be just fine for some applications, and not fast enough for others. 10,000 devices would take 10mS.
You do the sums for where the devices are (let's say) the LED panels for the advertising around sports grounds, or the LED panels of a live video display.
" each slave in the chain will act as the master for the following slave ".
That's poor use of language. Each slave is the data source for the following slave. However the master is what controls what happens and when. The master function stays with the master, in contrast to some busses where the 'master' function can be passed around.