It has changed over time.
With the release of Diamond and Pearl, you can play 4 of each name.
That means for DP Unowns, you can play 4 of each.
You can also play 4 of the Unseen Forces Unowns in total. They are all named just "Unown".
The older, Neo, Unown are a bit tricky.
They have the letter as part of their name, but they also have printed on them "you can only have a total of 4 Unown in your deck".
The DP rule changes would over turn this since it is a rule change.