Disputatio:Index Regum Hispaniae
E Vicipaedia
[recensere] Hispaniarum
Hmm, looks like at least in the era of Philip III and IV, and presumably not just them, the title was Hispaniarum Rex. E.g. http://www.maravedis.org/felipe3.html --Iustinus 08:20, 27 Ianuarii 2007 (UTC).