'How many countries are represented by the British Union Jack'?

Answer 1

In current day, there is no such thing as the "British Union Jack".

The flag in question is the Union Flag which is the flag of the United Kingdom of Great Britain and Northern Ireland (UK for short). So the answer to how many countries it represents is four, England, Scotland, Wales and Northern Island.

However, the Union Flag itself is made up of three flags, as follows.

St George's Cross (England) a centered red cross on a White Field

St Andrew's Cross (Scotland) a White Saltire (a crux decussate or X-shaped cross) in a Blue Field

St Patrick's Cross (Ireland) Red Saltire (a crux decussate or X-shaped cross) in a White Field

This flag dates back to the Union of Great Britain and Ireland from 1801.

The flag has mistakenly been called the Union Jack for many years; however, this has been tolerated and will probably never change.

The only time the Union flag should be called the Union Jack is when it is being flown from the Jack staff of a ship (The flagpole at the bow of a ship). In which case, any flag flown from a Jack Staff is referred to as a Jack.
Originally it was just England, Scotland and Wales. The cross of St George (England) over the Cross of St Andrew also known as the Saltire (Scotland), and the cross of St David (Wales) - then in 1801 the Saint Patrick's Saltire (Ireland) was added to make up the current flag.