We have a scenario where up to 250 alarms could go off at the same time. During our testing we noticed that it seemed like not all 250 alarms would go off (Maybe half would go off). However, when the alarms are tested individually they always work.

Is there any limit to how many alarms can go off at the same time? 

• Cycle and Response Times of the S7-400 or CPU 410 cycle and reaction times
• the Setting in the HW Config about the Scan cycle load from communications
• single or redundant mode of the AS
• single or redundant mode of the OS Server
• is there additional AS-AS communication

Check the parameter of the maximum communication jobs in your hw-config, marked in this FAQ:

https://support.industry.siemens.com/cs/de/en/view/604010

I remember there is a PCS7 manual where it is described what settings are recommended for a PCS7 AS.

In my PCS7 projects I've counted the number of Alarm_8 instances in my program, an set the number to a value slightly above when possible by the CPU limits.