Several nations have special event stations with call signs longer than normal. I know of one nation which issues 3-character prefixes (Eswatini), but those are all 3x2 call signs (3DAnxx). Australia until recently issued 2x4 calls (VKnFxxx) but stopped a little while ago. The USA once issued multiple number calls for special events IE W100AW. The longest I know of would have to be the STAYHOME call signs IE OH2STAYHOME which would be a 2x8 call sign, but are special events. Portable call signs such as VK6Fxxx/3DA0 could be a long call sign, but isn't issued per se. If that counts, a state QSO party call such as AI7OW/KITSAP would also qualify.
UPDATE: Guinea issues 3x1 call signs along with 2x1 and 2x2, 3XYn?