From The Truth About Guns:

It’s 10 rounds for handguns, and 5 rounds for rifles – except that restriction is on magazines themselves, not guns they’re inserted in, and not on how you use the magazine (the crime is “possession of a magazine capable of holding N rounds”, not “loading a magazine with N rounds” or “inserting the magazine with N rounds into gun X”).

https://youtu.be/FbZhYEVnXlA