The numbers reset in England - the normans didn't invade Scotland. James was known as James the sixth and first in Scotland. So he would in all likelihood have ben Constatine the IV and I.
The rules have since changed, after protests in Scotland in the 1950s that the new Queen should not be called Elizabeth II in Scotland, as the previous Elizabeth had not been Queen of Scotland. Now, any monarch gets the higher number from the English and Scottish sequences, starting with William the Conqueror (William I) and the quasi-mythical 5th-century Pictish King Drest I respectively. So, for instance, another Richard would be Richard IV in both countries- but a David would be David III, an Alexander would be Alexander IV, and a Drest would be Drest XI!
Isn't there a rule that there can never be another King John?
No- John, along with Stephen, Philip and Charles, is a name that is unlikely to be repeated due to its bad record, but there's no rule. The only name that there's a rule against using is Albert, as Queen Victoria said that there should never be a King Albert after Prince Albert died. Her eldest son, Albert Edward, took the throne as Edward VII rather than Albert I.