The Guardian) that Prince n of Cambridge will, all being well, eventually become the forty-third king since William the Conqueror. However, said invader was never king of Scots, and the new prince is as much a prince of Scotland as he is of England.
Turning to the list of Scottish monarchs in the trusty Oxford Dictionary of National Biography, copying it to a spreadsheet (purely for arithmetical reasons) and deleting unwanted copy, including sub-headings, and a second reign (Donald III, part two, 1094-1097), it appears that Prince n of Cambridge will be the sixty-third king or queen of Scots since Kenneth I/Cináed mac Alpin became king of the Picts as well of the Scots of Dalriada in 842. This excludes figures such as Amlaibh, included after or alongside Indulf in the Wikipedia list, as well as Edward Balliol who reigned in opposition to David II in the fourteenth century, but includes Margaret the Maid of Norway, the 'lady of Scotland' or queen-designate in ODNB terminology.
Kenneth has over two centuries' advantage over William I of England, however, so picking Malcolm III/Mael Coluim Ceann Mór/Malcolm Canmore (reigned 1058-1093), the twentieth on the list, as William's nearest contemporary, this makes the prince the potential forty-fourth king of Scots since Malcolm III inaugurated the continuous succession of Scots kings descended from his father Duncan I (reigned 1034-1040).