Limitation of nr of cp's in rack

In typical S7−300 configurations, the simultaneous operation of up to 4 CPs of the
same type has been tested successfully. The actual number of SIMATIC NET CPs
that can be operated at the same time is determined by the system (for example
by the CPU resources).
The connection resources available in the CPU can result in a further limitation.
The load on the CPU resulting from communication jobs may also represent a
further restriction. The following factors should be noted:
 Execution Time of the Blocks:
For communication between the S7-300 CPU and SIMATIC NET CPs, blocks
(FCs/FBs) are necessary. How often these blocks are called depends on the
number of connections or the number of SIMATIC NET CPs. Depending on the
amount of data transmitted, every block call extends the time required by the
user program.
 Data conversion:
It may also be necessary for the information to be converted before
transmission or after reception.