No hard limit that I am aware of, but a "soft" limit could be inability of the single SND/IRQ core to empty the ring buffers of so many interfaces in a timely fashion, leading to RX-DRPs as shown by netstat -ni. Also assuming that SecureXL is enabled, automatic interface affinity will spread out SoftIRQ handling across the 2/2 split (2 physical cores) in your situation but only if there is enough traffic to justify doing so. All SoftIRQ processing starts on CPU 0 and can then be moved around as traffic warrants, looks like in your lab setup there is not enough traffic present for any SoftIRQ processing to get moved off CPU 0.
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