Hi @JulianH
There is no recommended maximum for subinterfaces. There are only the specification max numbers for interfaces and subinterfaces which are the following for a PA-3220:
802.1q tags per device
4,094
802.1q tags per physical interface
4,094
Max interfaces (logical and physical)
4,096
As these interfaces are used in PAN-OS to decide where to forward traffic and this is all done in hardware you will not see a performance impact even if you go way above your currently 20 subinterfaces. I personally did not use the interfaces until the specified max so far but at least with more than hundret subinterfaces the firewall forwards the traffic as if there would be only one or two interfaces.
Hope this helps,
Remo
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