Regnal numbers are given to queens who reigned, not to the spouses of reigning kings. Queen Elizabeth II's mother, for example, would be entirely correctly also called "Queen Elizabeth" but during her life was generally referred to as the Queen Mother to avoid confusion, given that she ended up living until 2002. (This is ultimately a question for style guides. You can see The Telegraph's explanation here: https://www.google.com/books/edition/The_Daily_Telegraph_Style_Guide/kDvLAgAAQBAJ?hl=en&gbpv=1&dq=queen+mother+styling+avoid+confusion&pg=PT28&printsec=frontcover . The Times' style guide on how to refer to her disagrees and thinks that "the Queen Mother" remains fine and clear.)

Notably, there is some minor controversy over Elizabeth II's regnal number in Scotland, because Queen Elizabeth I did not rule Scotland. Scottish ire at this fact has mostly focused on the "E II R" design of post boxes. Essentially, regnal numbers are part of the "royal prerogative"--that wonderfully un-specified part of the British unwritten constitution where the King or Queen exercises power at his or her sole discretion.