Recent re-education on my part of NEC required over-current-protection (OCP) for solar panels yields that for less than or equal to 2 panels/strings in parallel, no OCP is needed.
2S, 6S, xS2P = none of these need fuses/breakers. As soon as you parallel 3 or more, you need a fuse/breaker on each parallel string.
TL;DR
you don't need one.
If you want to use one, the panel label likely lists a maximum fuse for a single string. If they are in series, you need only one. If they are in parallel, you could use 1 fuse that's 2X the panel stated fuse.