What is the maximum number of simultaneous BLE connections?
[disclaimer: I'm fairly new to BLE.  I've gone to some seminars.  I'm in the process of creating my first gizmo+app.]


  I'm aware that some centrals are only able to support a limited (usually &lt;8) number of connected peripherals. But is there any way around this?


Android 4.3 can support 4x GATT connections1, while Android 4.4 and up can support 7x.  This kind of change makes it look like a limitation in software rather than a fundamental limitation.

Having said that, I don't know how to increase the number of available GATT connections.  There may be workarounds specific for each OS.


  For example, is a situation possible where the peripherals are only visible to the specific central (i.e. other random devices can't see what they advertise), but still operate in an advertising mode when not being directly accessed? 


I don't think BLE or Android supports this kind of targeted advertising.  You could encrypt the advertisement packet's payload, though.


  [...] could alerts still be generated on the central device based on triggers from the advertising peripherals?


If your app successfully decrypts, it can generate an alert.

1 For the entire client device, as opposed to number of connections available per app.