This says three or more, as does countless posts I've seen:
To get the paper, click on the orange'' button at the top of the screen. This resource is intended to help the user understand when they need to add Over Current Protection Devices (OCPDs - Fuses or Breakers) to the solar panel array and what...
diysolarforum.com
2P being safe without needing to be fused is pretty much universally-agreed-upon.
And more than 3P is also universally agreed-upon as requiring a fuse or breaker on each string,
3P may or may not require fusing on each string depending on the precise specifications of the panel.
If you have a 3P array, a short within any panel can draw 2xIsc from the other two (unshorted) strings.
That 2xIsc will add to another Isc within the shorter panel itself for a total current of 3Isc at the short, 2 x Isc through the cables plus another Isc at the short itself.
Panels generally include a maximum fuse rating within the specifications, typically either 15A or 20A.
If your panel has a maximum fuse rating of 15A that translates to the panel manufacturer telling you that no more than 15A should ever flow through the output cable.
If 2Isc is > the maximum fuse rating, you will have a larger current glowing through that panels cables then recommended by the manufacturer and thus, there are no guarantees as far as safety.
So if the maximum fuse rating if a panel is > 2xIsc, a 3P string should almost certainly be safe without requiring fuses on each panel. But if not, it is recommended to fuse each panel/string:
https://www.cedgreentech.com/article/when-fuse-when-not-fuse
“Now let’s evaluate a system where there are three strings of modules connected in parallel to determine whether or not fusing would be a requirement. If a fault were to present itself in one string the other two healthy strings could force their currents through the faulted string. This implies that 2.5 times the Isc rating of the modules could be forced through the faulty string.
In many cases the integrated OCPD of the faulted string would be exceeded, requiring that there be fuses present on the output of each string to protect the wiring and equipment from damage.’
And in my above analysis I overlooked the 125% factor. Including that, you need a fuse whenever 2.5 x Isc exceeds the maximum fuse rating of the panel (which is most cases).
For 2S strings, the 125% factor does to trigger the need for fuses since that factor is already taken into account when the panel wires were sized (so the wires can safely handle a maximum 125%^2 156% of Isc whether being output from the panel through normal operation of coming into the panel from a parallel string because it has a short:
“Let’s consider a two string system. We know that each string in this condition could produce current that is 1.25 times that of their Isc rating. If a short were to occur the un-faulted string of modules could force its current through the faulted string.
In this scenario we know that the faulted string can already handle 1.56 times its Isc rating so it is still protected. No fuse necessary.”