Basically the standard defines it, by specifying how much a single receiving unit can load the bus (with resistance and capacitance), and how much load on a singly-terminated bus a single driving unit must be able to drive. The numbers are just selected so that a standard driver can drive a singly-terminated bus with ten standard receivers. Sometimes modern receivers do not load down the bus so much so if it presents half a load then you can have 20 half-loading receivers.
Also the wiring capacitance and resistance loads the bus, so long wires limit the number of receivers too. To calculate this you need to know the capacitance of the wire.
But that only applies to the RS-422 drivers drivers and buses.
Not to SSI encoder devices, that utilize RS-422 standard, and further define how they use the RS-422 signaling. As devices, they are meant to be connected so that there is one bus master, and one encoder, and you can't change that.
Your encoder already incorporates a RS-422 receiver for the clock bus, and since the encoder is meant to be the single receiver at the end of a bus, it provides a termination resistor. So you cannot have multiple encoders connected to the clock bus, and no matter how you multiplex the bus to many encoders, the encoder has to be at the end of the bus wire.
Also the encoders have RS-422 transmitters for the data. They are always active, driving the bus - at least no SSI bus application note says otherwise. With RS-422, only one transmitter can be active at one time, and the transmitter must also be at the start of the bus to avoid signal reflections from stubs.
So clearly, the SSI encoders are point-to-point devices meant to be on the end of the bus, not in the middle.
If you do use for example relays to mux between encoders, either you have to have the relays at the master end, and from each relay you run a cable to each encoder. If you want to have one chain of encoders, you have to have a relay at each encoder to select if the bus goes to this encoder or if the bus continues to the next relay.