Maximum number of alarms in storage (Alarm Manager)
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Unfortunately there is no official figure available.
However, with the "Make new alarm each time a condition gets active" setting disabled (which is default) the default capacity of 10.000 (=19 clicks on the "Next page" in an unfiltered alarm list where each page list 500 alarms) should be enough for most configurations.
I do, however recommend declaring separate alarm storage for system alarms since most installations care more for process alarms than system alarms. E.g. with a large amount of AC800 IO and the typical many many "underflow/overflow" alarms due to active, but unconnected IO loops the storage of process alarms might be threatened.
I also believe going a bit further than 10.000 is OK.
For "alarm history" I suggest using event lists; the event storage service can store up to 50.000 events per event category (10.000 is default setting). Unlike alarms, events are retained across system restarts, cold reset of controller, etc.