I was taught that two spaces should follow a colon or semicolon. Many rules have changed since I went to school. Can someone tell me how many spaces should follow a colon or semicolon? Thank you in advance!

The two-spaces ruling derives from when we used to use typewriters. Some have retained it after full stops (periods), but it is unusual to see it after colons or semicolons.

The answer is one space after a colon or a semicolon.