On a 100 Mbps PROFINET network in RT mode, you can typically support up to 100-200 IO Devices with Update Times of 10 ms without saturation, provided the topology stays reasonable (no more than about 4 hops).

Classical sizing of a 100 Mbps PROFINET RT network gives a cyclic bandwidth of roughly 10-100 kbps per device depending on the I/O count, so 100 devices at about 50 kbps each consume only 5 Mbps, i.e. 5 % of the 100 Mbps pipe, which leaves a comfortable margin. The real limit is not raw bandwidth but the number of hops and the cumulative store-and-forward latency they introduce. Beyond 200 devices, moving to 1 Gbps becomes cost-effective, and IRT requires a larger margin because time slots are reserved. Sizing tools such as Siemens SINETPLAN (free) or Hirschmann Network Sizing automate this calculation.

Always size against hop count and worst-case latency, not just average bandwidth: a network at 5 % load can still miss its Update Time if a critical telegram has to cross eight cascaded switches.