I am trying to create a number of different WiFI ssids on a tl-wr841n-v13 running 19.07. 4 seems to work, but 5 seems to fail. Four is probably enough for me, but I am curious about the origin of the limit.
Is this an OpenWRT limit, or something specific to the mt76 chipset?
Running iw list will tell you the valid interface combinations.
On most devices the number of concurrent AP interfaces is usually in the 4-8 (maybe 16, for very new/ highend ones) range, this is a hardware (firmware) limitation.