Among the Håkons/Haakons as well, there are two who have not been granted a regnal number:

Håkon I "the Good", 935-960
Håkon Toresfostre, king 1093-1094, in rivalry with his cousin Magnus III, and never assigned a regnal number
Håkon II Herdebrei ("the Broadshouldered"), 1157-1162
Håkon III Sverresson, 1202-1204
Håkon IV Håkonsson "the Old", 1217-1263
Håkon Håkonsson "the Young", king jointly with his father Håkon IV 1240-1257 - but never reigned alone, and was therefore not assigned a regnal number
Håkon V, 1299-1319
Håkon VI, 1355-1380
Haakon VII, 1905-1957

... and our current Crown Prince will some day be Haakon VIII.