the limit of how many devices can be connected to one RS485 line comes from the RS485 transceiver (the chip on the RS485 output) and on our PLCs it is 32. But be aware, that this is just a theoretical limit and the real limit depends on the current situation.
Second limit of the RS485 is the speed of the communication. Since RS485 is a half-duplex type of communication (master can speak only to one slave at a time), the more devices you have the lower speed of updates you will have.
If you have PLC with more RS485 ports, you can split the extensions into groups.