It does not even depend on the frequency of clock that much. It depends on a lot of factors and it is difficult to calculate.
First of all you do have to know what SPI frequency each device supports, as that is the ultimate limit for the clock for each device.
Then, you need to know, how much data you need to transfer from each device and how often, to figure out if it is even possible in theory.
So a few thermocouple bytes read a few times per second from 6 chips is not a problem as chips support up to 5 MHz clock and you don't have to use the fastest possible clock.
Now, comes the hard part, the electronics that may impose further limits, or requires more hardware to make it work. Most important factor is, how far apart are the devices, will they be on same PCB or far apart.
The MCU and the sensors have electrical specs. MCU output has drive strength to drive a certain waveform into a certain load. The sensors are a load and require a certain waveform to work properly. More specifically, sensor requires that clock edges are fast enough, and the load is capacitive. The MCU can drive output edges with certain current and the more capacitive load there is the edges are slower. MCU and sensor datasheets mention what the waveforms need to be, and what waveforms can be expected with a certain load.
More capacitance comes from wiring between chips. So it is possible that MCU can drive the load of six sensors in theory, but due to additional load from wiring, the edges may become too slow.
If the SPI bus needs to travel outside a small device, it likely requires extra bus transceivers to work.