In a Category 4 architecture, a series connection of emergency stop pushbuttons is allowed because
the emergency stop is only a secondary safeguard.
It is assumed that the emergency stop will only be operated in hazardous situations - and even then only one emergency stop pushbutton will be operated.
As part of the risk assessment in accordance with the industrial safety regulations, the operator must establish the intervals in which the safety functions must be inspected and the format that inspection will take.
Where is the limit in this case?
The limit lies in the required performance level. This is because the series connection of multiple safety components worsens the overall result. If you can reach the required PL by connecting the emergency stop pushbuttons in series, then the number is not inappropriate.
Is every switch rated as an SRP/CS in the context of the standard EN ISO 13849?
In the safety assessment you can connect all emergency stop switches in series, i.e. you only need to create one SRP/CS.
However, as described above, the series connection may worsen the PL and so it may be possible to create several SRP/CS.